Caramelized Onion Tart (Tarte à l’Oignon Alsacienne)

With its buttery, crisp crust and a rich filling of sweet caramelized onions in a savory custard, this Caramelized Onion Tart is a cherished gem from the Alsace region.

This rustic and comforting dish delivers a delightful balance of sweet and salty flavors, and is perfect as an elegant appetizer, a light lunch, or a satisfying dinner. Indulge in the timeless charm of this Eastern French classic!

What is a Caramelized Onion Tart (Tarte à l’Oignon Alsacienne)?

This French Onion Tart, known as “Tarte à l’Oignon” or “Zwiebelkueche”, is a cherished specialty from Alsace in eastern France. This classic dish is a staple in the menus of local winstubs (Alsatian bistros) and is equally popular in home kitchens.

As with many traditional recipes, there is no single definitive version of the Alsatian Onion Tart. Every chef and home cook like to add their own twist on it, with their cheese, herb or spice of choice. However, most variations share 3 essential components: a crisp and buttery pie crust, a generous amount of caramelized onions and a savory custard poured over the top.

1. The pie crust: Like for most quiches and savory tarts, I always go for a “pâte brisée”: a sturdy and buttery French crust that’s perfect for holding a soft filling.
2. The caramelized onions: As per local tradition, the onions are caramelized slowly in butter – and white wine. Why? Because Alsace is renowned for producing some of the finest white wines in the world, including Gewurztraminer. Thanks to the region’s very cold winters, the grapes develop a natural sweetness, resulting in wines that are often sweeter and deeply aromatic. These sweeter white wines are perfect for caramelizing onions, enhancing their natural sugars and transforming them into something irresistibly sweet and melt-in-your-mouth delicious.
3. The savory custard: The custard is made from eggs, milk and cream. It is seasoned with black pepper and caraway seeds. Caraway, known in French as “carvi”, is popular in Alsatian cuisine. It is often used to flavor dishes and commonly found in Munster, a flagship cheese of Alsace. This sprinkle of caraway is optional, but I think it brings out the true Alsatian flavors of this tart.  

Ingredients you’ll need for this Caramelized Onion Tart

Now let’s take a closer look at the ingredients you need to make this delicious, rustic tart in your own kitchen.

For the pâte brisée crust

1. Flour. All-purpose flour is the way to go.
2. Salt. A pinch of salt to heighten the flavors.
3. Butter. Use cold, cubed, unsalted butter for best results in this crust recipe. I like to cube the butter, lay the cubes on a plate and stick the plate back into the fridge for at least 1 hour before I start making the crust. This ensures the butter cubes are perfectly cold when they’re added to the flour.
4. Water. About ¼ cup (60ml) of ice-cold water is needed to bind the ingredients together. You may need a little less, so add the water one tablespoon (15ml) at a time.

For the filling

1. Onions. We need 3.3lbs (1.5kg) of onions for this recipe! Yellow onions are the best choice. They turn utterly sweet and melty when caramelized. In Brittany, we have pink onions, also known as Rosé de Bretagne, which are sweet onions (similar to yellow) with a light pink outer skin. I sometimes use them too (pictured here). 
2. Butter. A generous amount of butter is key to caramelize these onions well.
3. White wine. Opt for a sweeter white wine to helps build a nice caramelization. My recommendations are an Alsatian Gewurztraminer or sweet Chardonnay (more affordable). That said, a drier white wine like a Sauvignon Blanc or Pinot Gris will do the job nicely too. The alcohol will completely burn off after the one hour of cooking.
4. Milk and cream. ½ cup of milk and ½ of heavy cream is the base of the custard filling.
5. Eggs. 3 large eggs are used in the filling. They need to be at room temperature so take them out of the fridge one hour before you start cooking.
6. Salt, pepper and caraway. Season to taste!
7. Gruyère Cheese. Although not traditionally used in a French onion tart, I like to add about 1/2 cup (50g) of Gruyère cheese to the filling. It adds some richness with a salty kick that contrasts well against the sweet caramelized onions. Emmental works well too.

Tips & Tricks

• Make your own crust, if you have time.  

Although you can use a store-bought pie crust for convenience, making your pie crust from scratch will go a long way. The recipe below only requires about 15 minutes of time. It guarantees a flaky, buttery crust that is the perfect vessel for the creamy custard and soft onions.

It is essential to use cold butter and ice-cold water to make the pie crust. Cold butter – not room temperature – will distribute into the flour without softening. And ice-cold water will bind the ingredients together without melting the butter. This ensure little bits of butter are still present in the dough once rolled out, which makes for a flaky crust. 

This recipe will make enough for a high-sided 9-inch (22.9cm) or 10-inch (25.4cm) pie crust, plus a little extra if you want to make an individual tart on the side – or keep it in the freezer for another project (well-wrapped in plastic film). You definitely won’t regret taking the time and minimal extra effort to make it. 

• The Caramelized onions

The caramelized onions are the stars of this recipe, so make sure they are done properly. Caramelizing onions, by slowly cooking them in butter and deglazing in white wine until they are golden and melty, extracts the natural sugars from the onions and intensifies their flavor.  The trick is to not stir too often to allow for the caramelization to happen at the bottom of the pan. I find that stirring onion every 5 minutes is good enough.

For this Caramelized Onion Tart recipe, the onions do not need to be caramelized too much. Reach for a golden color, not brown. The process of caramelizing onions should take about 45 minutes to 1 hour, on medium-low heat.

• The Cheese

For the filling, be sure to use a hard Swiss-made cheese such as Gruyère or Emmental – both of which should bear an AOP (Appellation d’Origine Protégée) seal. This sign on the label shows the cheese was made according to strict government mandated rules and quality standards. Both cheeses are subtly salty with lovely nutty notes. They are pricier than cheddar, but well worth it when used in the filling of this tart.

Frequently Asked Questions

How long can you keep this Caramelized Onion Tart?  

After making (and enjoying) this tart– if you have any leftovers – you can store them in an airtight container in the fridge for up to 4-5 days. I don’t recommend re-heating it in the microwave, which can make the crust soft. Instead, reheat the tart in the oven for about 5-10 minutes at 350°F (180°C). 

Can you freeze the tart?

Yes, absolutely. Once completely cool, you can store it in a (freezer-friendly) airtight container and place it in the freezer for up to 3 months. To reheat, let the tart thaw in the fridge before heating it through in the oven. 

What do you serve with this Caramelized Onion Tart?

In Alsace, this tart is traditionally served in thin slices as a starter, accompanied by a fresh leaf salad. It also works wonderfully as a satisfying lunch paired with sides like carrots, green beans, or even a hearty dish such as cauliflower gratin.

Personally, my favorite way to enjoy a slice of onion tart is for dinner, served alongside a comforting bowl of warm soup.

Helpful swaps and notes: 

• Caramelizing the onions low and slow is key here. Take your time! So, this recipe is best made on weekends, when you have more time to stay in the kitchen.
• For caramelizing the onions, I like to use butter and white wine, which both provide richness and classic earthy flavors. Some other recipes call for beef stock instead of white wine. Although this isn’t the traditional recipe, using beef stock works great as a slightly more affordable alternative.
• The amount of filling in this recipe is good for a high-sided 9-inch (22.9cm) or 10-inch (25.4cm) pan. Do not fill the crust to the rim: the filling will puff up as it bakes, and deflate slightly as it cools down. If you have some filling left, you can make an individual tart on the side with the extra crust you have. I recommend using a pan with a removable bottom, which will support the crust when lifting your tart.

More rustic French recipes you may like: 

• Classic French Quiche Lorraine
• Spinach and Cheese Quiche
• French Style Cottage Pie (Hachis Parmentier)
• French Chicken And Mushroom Pie (Tourte)
• French Stuffed Cabbage Rolls (Choux Farcis)
• French Cabbage and Sausage Soup (Soupe Paysanne)
• Classic French Lentil Soup
• Classic French Onion Soup
• Braised Pork Loin with Prunes (Porc aux pruneaux)

Caramelized Onion Tart (Tarte à l’Oignon Alsacienne)

Print Recipe

Serves: 8-10 Prep Time: 20 Minutes Cooking Time: 2 Hours 2 Hours

Nutrition facts: 200 calories 20 grams fat

Rating: 5.0/5

( 1 voted )

Ingredients

For the pie crust:
2 cups (260g) all-purpose flour
1 tsp salt
2/3 cup (150g) cold unsalted butter, cut into small cubes
4 tbsp (60ml) ice cold water

For the filling:
3.3 lbs (1.5 kg) onions
1/2 cup (115g) unsalted butter
2/3 cup (158ml) white wine
½ cup (125ml) whole milk (whole or 2% m.f)
½ cup (125ml) heavy cream
3 large eggs, at room temperature 
1 tsp caraway seeds
¼ tsp salt
¼ tsp black pepper
1/2 cup (50g) Gruyère cheese, grated

Instructions

1. Make the crust. Mix the flour, salt and butter with your fingers or pulse in a food processor, until you get a crumbly consistency and pea-size bits of butter are still visible. Add the water, one tablespoon at a time, and mix until the dough roughly comes together into a ball; do not overmix. Add a few more splashes of water if you feel the dough is too crumbly/dry. Wrap the dough in plastic wrap and chill for at least 30 minutes.
2. Pre-heat your oven to 350°F (180°C) with a rack in the middle. 
3. Line with parchment paper a 9-inch (22.9 cm) or 10-inch (25,4 cm) pan. Transfer the dough onto a floured working surface and roll it out to a 12-inch (31cm) circle. Transfer the crust to the prepared tart pan. Cut any excess crust that hangs over the edges and crimp the edges (optional). Poke the bottom of the crust all over with a fork. Line the top of the crust with foil or parchment paper and place pie weights or dried beans over top, to keep the pie crust from puffing when baking.
4. Bake the crust for 15 minutes. Carefully remove the parchment paper filled with weights and set the crust aside.
5. Caramelize the onions. Melt the butter in a large frying pan over medium-low heat. When the butter is sizzling, add the onions slices and stir to coat with the butter. Using a spatula or wooden spoon, spread the onions evenly onto the pan. Let the onions cook for 15 minutes, stirring every 5 minutes. After 15 minutes, when the onions should start to dry out, add the white wine. Stir to coat and continue cooking for about 30 more minutes, until the onions are melty and golden. Set aside.
6. Make the filling. In a large mixing bowl, whisk together the eggs, the heavy cream and the milk. Add caraway, salt, black pepper and grated cheese.
7. Assemble the tart. Scatter the caramelized onions evenly onto the bottom of the pre-baked crust. Pour the prepared liquid filling over top.
8. Bake for 1 hour at 350°F (180°C) , or until golden. The middle will be slightly wobbly and puffy, and will firm up and deflate as it cools down. Transfer onto a cooling rack and let cool at least 15 minutes before serving. Enjoy it warm or at room temperature. 

Notes...

Store leftovers in airtight container in the fridge for up to 4-5 days. Reheat in the oven for about 5-10 minutes at 350°F (180°C).  Freeze in an airtight container for up to 3 months. To reheat, let the tart thaw in the fridge before heating it through in the oven.