Savory Caramelized Onion Tart

This hearty onion tart has been my go-to recipe for years, perfect for those evenings when I want something that feels special without hours in the kitchen. The combination of sweet caramelized onions nestled in a buttery crust creates a dish that's both rustic and elegant enough for any occasion.

I first made this tart for a last-minute dinner party when I had little in my pantry besides onions and cheese. The compliments were so enthusiastic that it's now become my signature dish when entertaining friends.

Ingredients

For the crust

• All-purpose flour: Forms the base of our buttery crust, providing structure
• Cold unsalted butter: Creates those essential flaky layers when it melts in the oven
• Egg: Adds richness and helps bind the dough together
• Salt: Enhances all the flavors in the crust
• Cold water: Helps bring the dough together without overworking the gluten

For the filling

• Large onions: Are the star component sweet and mellow when caramelized
• Unsalted butter: Adds richness to the onions as they slowly cook down
• Heavy cream: Creates that luxurious custard texture that holds everything together
• Eggs: Provide structure to the filling helping it set beautifully
• Hard cheese: Brings savory depth use Gruyère for authentic flavor or any good melting cheese you have
• Salt and pepper: Balance and enhance all the flavors
• Nutmeg: Adds a subtle warmth that complements the sweet onions perfectly

Step-by-Step Instructions

Prepare the Pastry:

Combine flour, salt, and cold butter using your fingertips until the mixture resembles breadcrumbs. This creates pockets of butter that will make your crust flaky. Work quickly so the butter stays cold. Add the egg and just enough cold water to bring the dough together. Avoid overworking which can make the crust tough.

Chill the Dough:

Form the dough into a ball, wrap in plastic, and refrigerate for at least 30 minutes. This relaxes the gluten and firms up the butter, ensuring your crust will be tender and flaky. You can prepare this a day ahead if needed.

Caramelize the Onions:

Slice onions thinly and melt butter in a large skillet over medium heat. Add the onions and cook slowly for about 20 minutes, stirring frequently. Watch for that golden brown color developing but avoid burning. The slow cooking develops incredible sweetness that makes this tart special.

Prepare the Custard:

Whisk eggs and cream together until smooth, then add grated cheese, salt, pepper, and nutmeg. This custard will set around the onions, creating a silky texture that contrasts beautifully with the crisp pastry.

Assemble and Bake:

Roll out your chilled dough on a lightly floured surface and transfer to your tart pan. Press gently into the corners and trim excess. Prick the base all over with a fork to prevent bubbling. Spread the caramelized onions evenly across the base and pour the custard mixture over them. Bake until golden and just set in the center.

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You Must Know

My favorite moment with this recipe was surprising my mother with it during a summer visit. She had always been the cook in our family, but after one bite of this tart, she asked for the recipe. Now it has become part of our family traditions, appearing at nearly every gathering.

Make Ahead Options

The beauty of this onion tart lies in its flexibility. You can prepare components in stages to fit your schedule. The pastry dough can be made up to three days ahead and stored in the refrigerator. For even longer storage, freeze the raw dough for up to a month. The onions can be caramelized a day ahead and stored in the refrigerator, which actually improves their flavor. You can even assemble the entire tart and refrigerate it unbaked for several hours before baking directly from the refrigerator, just add 5 minutes to the baking time.

Perfect Pairings

This onion tart shines when served with the right accompaniments. For a light lunch, pair it with a simple green salad dressed with a bright vinaigrette to cut through the richness. If serving as a dinner, it works beautifully alongside roasted vegetables or a bowl of soup. Wine lovers will appreciate how well this tart pairs with a crisp white wine like Pinot Grigio or a light red like Beaujolais. The caramelized onions have a natural sweetness that harmonizes with these wine varieties without overwhelming them.

Troubleshooting Tips

Achieving the perfect onion tart can sometimes present challenges. If your crust shrinks during baking, try chilling the lined tart pan for 15 minutes before baking. For soggy bottom prevention, you can blind bake the crust for 10 minutes before adding fillings. If your onions are browning too quickly rather than caramelizing slowly, lower your heat and add a tablespoon of water to the pan. And remember, the tart should be just set in the center when done, with a slight wobble that will firm up as it cools.

Recipe FAQs

→ Can I make the pastry crust ahead of time?

Yes, the shortcrust pastry can be made up to 2 days in advance. Keep it wrapped tightly in plastic wrap in the refrigerator. You can also freeze it for up to a month. Just thaw overnight in the refrigerator before rolling out.

→ What's the best way to caramelize onions?

For proper caramelization, cook the onions low and slow. Melt butter in a pan over medium heat, add thinly sliced onions, and reduce to medium-low. Stir occasionally for about 20-25 minutes until they become golden brown and sweet. Be patient—rushing this process will result in sautéed rather than caramelized onions.

→ What cheese works best in this tart?

Gruyère is traditional and offers a nutty flavor that pairs beautifully with caramelized onions. Alternatives include aged cheddar, Emmental, or Comté. For a milder flavor, try a mixture of mozzarella and Parmesan. The key is using a cheese that melts well.

→ Can I make this tart vegetarian?

This tart is already vegetarian! Just be sure to check that your cheese is made with vegetarian rennet if that's a concern. To make it completely plant-based, you could substitute vegan butter, plant-based cream alternatives, and vegan cheese, though the texture and flavor will differ from the original.

→ How do I know when the tart is fully baked?

The tart is done when the filling is set (not jiggly in the center) and the top is golden brown. The crust should be firm and lightly browned around the edges. This typically takes 25-30 minutes at 180°C (350°F), but oven temperatures vary, so start checking at the 25-minute mark.

→ What can I serve with this onion tart?

A light green salad with a simple vinaigrette makes an ideal companion to balance the richness of the tart. For a more substantial meal, add a bowl of soup—tomato or pumpkin work wonderfully. In summer, serve with fresh sliced tomatoes dressed with olive oil, salt, and basil.