Tarte À L'oignon Recipe

Make dough

In a large bowl, whisk together 1.5 cups flour and 0.5 tsp salt. Add 6 tbs cold, cubed butter and use a pastry cutter or your fingertips to work the butter into the flour until mixture resembles coarse crumbs with some pea-sized pieces.

Stir in the optional egg yolk (if using). Add 3 tbs cold water, one tablespoon at a time, tossing gently until dough just comes together when pressed. Do not overwork.

Turn dough onto a lightly floured surface, form into a disk, wrap in plastic, and refrigerate for at least 30 minutes (up to 2 days).

Blind bake crust

Preheat oven to 375°F (190°C). On a lightly floured surface, roll chilled dough to about 12-inch circle and fit into a 9- or 10-inch tart pan with removable bottom. Trim excess. Line crust with parchment and fill with pie weights or dried beans. Chill 10 minutes to relax the dough.

Bake crust with weights for 15 minutes, remove weights and parchment, then bake 6–8 more minutes until the bottom sets and is lightly golden. Remove from oven and let cool slightly. Reduce oven to 350°F (175°C) for final baking with filling.

Caramelize onions

While dough chills (or when crust is baking), heat 2 tbs butter and 1 tbs olive oil in a wide skillet over medium-low heat. Add 4 cups thinly sliced yellow onions and 1 tsp salt. Stir to coat.

Cook onions slowly, stirring every few minutes, for 25–35 minutes until deeply golden and soft. Sprinkle 0.5 tsp sugar about 10 minutes in to help caramelization. If onions start to stick or brown too fast, lower heat and add a splash of water.

When onions are soft and caramelized, increase heat to medium and add 0.25 cup dry white wine or dry sherry to deglaze the pan, scraping up browned bits. Stir in 1 tsp fresh thyme leaves and 0.5 tsp freshly ground black pepper. Cook until liquid has evaporated, then remove from heat and let cool slightly.

Prepare custard

In a mixing bowl, whisk together 3 large eggs, 1 cup heavy cream, 0.25 cup milk, 0.5 tsp salt, 0.25 tsp black pepper, and 0.125 tsp freshly grated nutmeg until smooth.

Assemble tart

Sprinkle 3/4 cup of the grated Gruyère evenly over the par-baked crust. Spread the caramelized onions in an even layer on top of the cheese. Pour the egg-and-cream mixture over the onions, letting it settle. Sprinkle the remaining 1/4 cup Gruyère and 2 tbs grated Parmesan evenly over the top.

Brush the exposed crust edge lightly with 1 tsp olive oil or melted butter to promote browning.

Bake

Bake tart at 350°F (175°C) for 30–35 minutes, or until custard is set (it should jiggle slightly in the center but not be liquid) and the top is golden. If the crust browns too quickly, tent loosely with foil during baking.

Remove tart from oven and let rest on a wire rack for 10–15 minutes to finish setting.

Garnish the tart with fresh thyme sprigs. Run a knife around the edge, remove from tart pan, slice into 6–8 pieces and serve warm or at room temperature.

Leftovers: refrigerate covered for up to 3 days. Reheat gently in a 325°F (160°C) oven for 10–12 minutes to refresh.