Onion Tart Recipe

Make dough

In a medium bowl or food processor, combine 1.5 cups all-purpose flour and 1/2 tsp salt. Cut or pulse in 6 tablespoons cold unsalted butter until mixture resembles coarse crumbs with some pea-sized pieces remaining.

Sprinkle 3 tablespoons cold water over the flour-butter mixture and gently mix just until the dough holds together when pinched. Add another tablespoon of water if it seems dry, but avoid overworking.

Form the dough into a disk, wrap in plastic, and chill in the refrigerator for at least 30 minutes (up to 2 days).

When chilled, lightly flour a work surface and roll the dough into a circle about 11-12 inches in diameter. Transfer to a 9- or 10-inch tart pan with removable bottom, pressing the dough into the edges and trimming the excess. Crimp or smooth the edges.

Chill the lined tart pan in the fridge for 15 minutes while you prepare the filling. If desired, beat 1 egg and brush the crust for an egg wash before blind-baking.

Preheat the oven to 375°F (190°C). Line the crust with parchment and fill with pie weights or dried beans. Blind bake for 15 minutes, remove weights and parchment, then bake 6-8 minutes more until the crust is just starting to color. Remove from oven and reduce oven temperature to 350°F (175°C).

While the crust chills, heat a large skillet over medium heat. Add 2 tablespoons olive oil and 1 tablespoon unsalted butter.

Add 4 cups thinly sliced yellow onions and stir to coat. Cook over medium-low to medium heat, stirring occasionally, until onions are soft, deeply translucent and lightly caramelized, about 25-30 minutes. If the pan gets too dry, add a splash of water.

About halfway through cooking, add 1/2 teaspoon sugar to encourage caramelization and 1 teaspoon salt. Continue to cook and stir, scraping any browned bits into the onions.

When onions are richly golden, stir in 1/2 teaspoon ground black pepper and 1 tablespoon fresh thyme leaves (or 1/2 tsp dried). Taste and adjust seasoning. Remove from heat and let cool slightly.

In a bowl, whisk together 2 eggs and 3/4 cup heavy cream until smooth. Stir in 3/4 cup grated Gruyère, 1/4 cup grated Parmesan (if using), and 1/8 tsp freshly grated nutmeg. Season lightly with a pinch of salt (remember the onions and cheeses are salty).

Arrange the caramelized onions evenly over the pre-baked crust. Pour the egg-cream-cheese mixture over the onions, allowing it to settle into the tart. The filling should come nearly to the top but not overflow.

Place the tart on a baking sheet and bake at 350°F (175°C) for 25-30 minutes, until the custard is set (a slight jiggle is fine) and the top is lightly golden.

Remove from oven and let cool on a rack for 10-15 minutes. Run a knife around the edge, unmold from the tart pan, and transfer to a cutting board. Garnish with a few extra thyme leaves if desired.

Slice and serve warm or at room temperature. Leftovers keep refrigerated for up to 3 days; reheat gently in a low oven.