Caramelized Onion Tart with Gruyere

This tart is a vehicle for two of my favorite things – caramelized onions and Gruyere cheese. Caramelized onions are super sweet, thanks to the cooking process which takes the time to allow the natural juices and sugars to release and reduce, resulting in a squidgy caramelized heap of onion. Gruyere cheese is a wonderful Swiss melting cheese (and a key ingredient in fondue) which is nutty and piquant and tames the sweetness of the onions. Serve this tart as a light meal, or cut into thin slivers and pass around as an appetizer.

Caramelized Onion and Gruyere Tart
Serves 8

Dough:
1 cup all-purpose flour
1/2 teaspoon salt
6 tablespoons chilled unsalted butter,  cut in 1/4-inch cubes
3 tablespoons ice water

Filing:
2 tablespoons olive oil
2 pounds yellow onion, peeled and thinly sliced
1 teaspoon salt
2 tablespoons Calvados
1 teaspoon freshly ground black pepper
2 ounces finely grated Gruyère cheese
1 teaspoon fresh thyme, plus extra for garnish
1 egg, slightly beaten

1. Prepare the crust: Combine the flour and salt in a food processor and pulse once or twice to combine. Add the butter and pulse until the dough resembles coarse meal, with some pieces of the butter visible. Add the water and pulse once or twice – just until the dough comes together, adding another tablespoon of water if necessary. Dump the dough onto a work surface and form it into a disk. Wrap in plastic and refrigerate for at least 1 hour.
2. Preheat the oven to 375°F.
3. Heat the olive oil over medium heat in a deep skillet or pot. Add the onions and salt. Cook the onions, stirring occasionally, until they are golden brown, soft and squidgy, about 30 minutes. Add the Calvados and black pepper and cook until the liquid evaporates, about 2 minutes, stirring frequently. Remove from the heat and cool slightly.
4. Roll out the dough to fit in the bottom and up the sides of a 10-inch round tart tin. Sprinkle half of the cheese over the bottom of the tart. Spread the onions in the shell and sprinkle the thyme over the onions. Brush the exposed crust rim with the egg wash. Sprinkle the tart and crust with the remaining cheese.
5. Bake the tart until the crust is firm and golden and the onions are deeply colored. without blackening, about 30 minutes. Remove and cool slightly. Serve slightly warm or at room temperature garnished with additional thyme.