- 1 1/4 cups all-purpose flour
- 8 tablespoons (1 stick) unsalted butter , very cold, cut into 1/2-inch pieces
- 1/2 teaspoon salt
- 1 teaspoon sugar
- 2 to 3 tablespoons nonfat milk , very cold
- 1 1/2 pounds beefsteak tomatoes , sliced 1/4 inch thick
- 2 tablespoons olive oil
- 1 small onion , peeled and sliced into thin rings
- 1/2 cup heavy cream
- 2 teaspoons fresh thyme leaves
- 2 cloves garlic , minced
- 1 teaspoon kosher salt
- 1/2 teaspoon freshly ground black pepper
- 1 cup grated Asiago cheese (about 4 ounces)
Add milk 1 Tbsp. at a time, pulsing as you go, until mixture forms a ball. Remove dough from processor and shape into a 5-inch disk. Wrap in plastic and refrigerate at least 1 hour (or wrap again in foil and freeze up to 6 weeks).
Preheat oven to 400°. On a floured board, allow dough to sit, unwrapped, until just pliable, about 15 minutes. With a floured rolling pin, roll dough into a large circle, moving it a quarter turn with every roll to make sure it's not sticking underneath (dough should be 1/4- to 1/8-inch thick.)
Ease dough round into a 9-inch tart pan with a removable bottom. Press tightly into the sides of the pan (dough should exceed the edge by 1/2 inch). Using the rolling pin, cut excess dough against the tart pan to create a perfectly even edge. Chill 30 minutes before cooking.
With the tines of a fork, poke holes throughout the bottom of the crust. Cover with foil; top with pie weights or uncooked beans or rice. Bake 15 to 20 minutes, or until edge of crust is light golden. Remove crust from oven and carefully remove weights and foil. Return to oven to bake 5 to 10 minutes more. Cool on rack. Keep oven on at 400°.
To make filling: Meanwhile, place tomato slices on a paper-towel-lined surface and let sit 5 minutes, flipping once. In a large skillet, heat 1 Tbsp. olive oil over medium heat. Add onion and cook until golden brown, 7 to 12 minutes; transfer to a plate. Add remaining tablespoon olive oil to skillet and, in batches (careful not to crowd), sauté tomato slices until just golden, 2 to 3 minutes each side; transfer to plate with the onions. Add cream, thyme, and garlic to the remaining juices; increase heat and cook until liquid is reduced by half. Season with salt and pepper; add 3/4 cup cheese. Stir to combine.
Pour reduced liquid into prepared pie shell. Top with onions and tomatoes; sprinkle with remaining cheese. Cook until tart is hot and cheese is melted, 10 to 15 minutes.