Sweet Onion Tart

Ingredients

• 1.25 cups all-purpose flour
• 0.5 tsp fine sea salt
• 8 tbsp unsalted butter – cold, cut into cubes
• 4 tbsp ice water – ice-cold
• 2 tbsp unsalted butter
• 2 pounds sweet onions – thinly sliced
• 1 tsp kosher salt
• 3 eggs – large
• 1 cup heavy cream
• 0.25 tsp ground nutmeg
• 0.25 tsp black pepper – freshly ground
• 3 ounces Gruyère cheese – coarsely grated

Instructions

1. Make the pastry: In a mixing bowl, combine the all-purpose flour and fine sea salt. Cut in the cold, cubed unsalted butter until pea-sized. Sprinkle in the ice water and mix just until the dough comes together; form into a disk, wrap, and chill 30 minutes.
2. Prepare the shell: On a lightly floured surface, roll the dough to about 12 inches. Fit into a 9-inch tart pan with a removable bottom, trim the edge, and dock the base with a fork. Chill 15 minutes while you heat the oven to 375°F (190°C).
3. Blind-bake: Line the chilled shell with parchment and fill with pie weights. Bake 15 minutes, then remove weights and parchment and bake until lightly golden, 8–10 minutes more. Set on a rack to cool slightly.
4. Cook the onions: Melt the 2 tbsp unsalted butter in a large skillet over medium-low heat. Add the sweet onions and kosher salt; cook, stirring often, until very soft and pale golden without browning, 25–30 minutes. Let cool 10 minutes.
5. Make the custard: In a bowl, whisk the eggs, heavy cream, ground nutmeg, and black pepper until smooth.
6. Assemble: Stir the cooled mixture into the custard, then spread evenly in the warm tart shell. Sprinkle the Gruyère cheese over the top.
7. Bake until the filling is set around the edges with a slight wobble in the center and the top is golden, 25–30 minutes. Cool 10–15 minutes, remove the outer ring, slice, and serve warm or at room temperature.