Caramelized Onion Tart Recipe

Overview

This caramelized onion tart layers sweet, deeply browned onions over crisp puff pastry with tangy goat cheese, thyme, and a little Dijon mustard. The pastry bakes around the edges while the onions become soft enough to spread with a spoon.

The onions need time rather than high heat: cook them slowly until golden and jammy, then cool them briefly before assembling. Serve the tart warm or at room temperature with a green salad, or cut it into small squares for a brunch table.

Ingredients

• 2 tablespoons unsalted butter
• 1 tablespoon extra-virgin olive oil
• 3 large yellow onions, about 2 pounds total, thinly sliced into half-moons
• 1/2 teaspoon fine salt, plus more to taste
• 1 tablespoon balsamic vinegar
• 1 teaspoon fresh thyme leaves, plus extra for serving
• 1 sheet frozen all-butter puff pastry, about 8 ounces, thawed according to the package directions
• 1 tablespoon Dijon mustard
• 4 ounces soft goat cheese, crumbled
• 1 large egg, beaten with 1 teaspoon water
• 1/4 teaspoon freshly ground black pepper

How to make it

1. Heat the oven and prepare the pastry. Position a rack in the center of the oven and heat it to 400°F. Line a baking sheet with parchment paper. Unfold the thawed puff pastry onto the sheet. If it is thicker than about 1/8 inch, roll it lightly to an even thickness. With a sharp knife, score a 1/2-inch border around the pastry without cutting all the way through. Prick the center area several times with a fork, leaving the border unpricked.

2. Start the onions. Heat the butter and olive oil in a wide 12-inch skillet over medium heat. Add the onions and salt, then stir to coat. Cook for 10 minutes, stirring every few minutes, until the onions soften and begin to look glossy.

3. Caramelize slowly. Reduce the heat to medium-low. Continue cooking for 35 to 45 minutes, stirring every 5 to 7 minutes and scraping the bottom of the pan. If the onions begin sticking or browning too quickly, add 1 tablespoon of water and lower the heat. They are ready when they are deeply golden brown, very soft, and reduced to roughly 1 1/2 cups. The mixture should look moist but not watery.

4. Finish the onion mixture. Stir in the balsamic vinegar, thyme, and black pepper. Cook for 1 to 2 minutes, until the vinegar has evaporated and the onions look jammy. Taste and add a pinch more salt if needed. Transfer the onions to a bowl and let them cool for 10 minutes. Warm onions are fine; very hot onions can soften the pastry before it reaches the oven.

5. Build the tart. Spread the Dijon over the scored center of the pastry, keeping the 1/2-inch border clear. Scatter the goat cheese over the mustard, then spoon the onion mixture evenly on top. Keep the filling inside the border so the pastry can puff around it.

6. Brush and bake. Brush the pastry border with the beaten egg, avoiding the cut edge as much as possible. Bake for 22 to 28 minutes, rotating the sheet halfway through, until the border is fully puffed and deep golden brown and the underside feels crisp when carefully lifted. If the top is browning before the pastry is crisp, lower the oven to 375°F and continue baking for 5 minutes.

7. Rest and serve. Let the tart rest on the baking sheet for 10 minutes. Finish with extra thyme, then slice into rectangles or squares with a sharp knife. The center should be soft and savory, while the border should crackle lightly when cut.

Expert tips

• Slice the onions evenly so thin pieces do not burn before thicker pieces soften.
• Use a wide skillet. Crowded onions release moisture slowly and take longer to brown.
• Do not rush the caramelization by turning up the heat. Dark brown bits are useful, but black spots will make the filling bitter.
• If your puff pastry becomes warm and floppy, chill the assembled tart for 10 to 15 minutes before baking.
• For a milder, creamier tart, replace the goat cheese with 4 ounces of crumbled feta or shredded Gruyère. Gruyère will melt more evenly and taste less tangy.
• For a dairy-free version, use dairy-free puff pastry and replace the butter with another tablespoon of olive oil. Omit the goat cheese or use a dairy-free soft cheese.
• If the pastry center puffs dramatically, press it down gently with the back of a spoon after baking. Pricking the center thoroughly before baking also helps keep the filling level.

Serving and storage

Serve the tart warm or at room temperature with a crisp green salad. It also works as a starter when cut into narrow strips. For another savory onion-centered dish, compare the soft egg-and-potato texture of this Spanish tortilla recipe.

Cool leftovers within 2 hours, then refrigerate them in a covered container for up to 3 days. Reheat pieces on a baking sheet at 350°F for 8 to 12 minutes, until warmed through and the pastry has regained some crispness. A microwave will warm the tart but will make the pastry soft. The unbaked onion mixture can be refrigerated for up to 3 days; bring it close to room temperature before assembling so it does not chill the pastry excessively.

Frequently asked questions

Can I make the caramelized onions ahead of time?

Yes. Cook the onions through the balsamic-vinegar step, cool them, and refrigerate them in a covered container for up to 3 days. Let them sit at room temperature for about 20 minutes before assembling the tart.

Why are my onions not caramelizing?

The pan may be too crowded, the heat may be too low, or the onions may still be releasing moisture. Keep cooking uncovered and stir occasionally. A wide skillet helps the liquid evaporate; avoid adding more than a tablespoon of water at a time.

Can I use store-bought pie crust instead of puff pastry?

Yes, but the texture will be different. Use a 9-inch refrigerated pie crust, fit it into a tart pan or pie plate, prick the base, and blind-bake it at 375°F for about 12 minutes before adding the filling. Bake again until the crust is golden and crisp.

How do I stop the tart base from becoming soggy?

Cook the onions until they are jammy rather than wet, cool them briefly, and keep the Dijon, cheese, and onions inside the scored border. Baking on a preheated baking sheet can also help the bottom crisp more quickly.

Can I freeze caramelized onion tart?

The tart is best fresh, but cooled slices can be frozen in a single layer and then transferred to an airtight container for up to 1 month. Reheat them from frozen at 350°F until hot and crisp; the pastry may be slightly less flaky than when freshly baked.

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