- 3 cups unbleached all-purpose flour
- 1 1/2 sticks (3/4 cup) cold unsalted butter, cut into 1/2-inch cubes
- 1/2 teaspoon fine sea salt
- 8 to 10 tablespoons ice water
- 1 medium boiling potato (5 oz)
- 2 cups finely chopped onion (1 large)
- 2 tablespoons unsalted butter
- 3 large eggs
- 1/4 cup heavy cream
- 1 teaspoon finely chopped fresh thyme or 1/2 cup finely chopped fresh parsley
- 1/4 teaspoon fine sea salt
- 1/4 teaspoon black pepper
- 9 oz good-quality white Cheddar, coarsely grated (2 1/2 cups)
- Special equipment: a pastry or bench scraper; a 9-inch tart pan or pie plate
- Blend together flour, butter, and salt in a bowl with your fingertips or a pastry blender (or pulse in a food processor) just until most of mixture resembles coarse meal with some small (roughly pea-size) butter lumps.
- Drizzle evenly with 8 tablespoons (1/2 cup) ice water and gently stir with a fork (or pulse in processor) until incorporated.
- Squeeze a small handful: If it doesn't hold together, add more ice water, 1 tablespoon at a time, stirring (or pulsing) until incorporated, then test again. (Do not overwork mixture, or pastry will be tough.)
- Turn out mixture onto a work surface and divide into 6 portions. With heel of your hand, smear each portion once or twice in a forward motion to help distribute butter. Gather dough together with scraper and press into 2 balls, one slightly bigger than the other, then flatten into disks. Chill each disk, wrapped in plastic wrap, until firm, at least 1 hour.
- Peel potato and cut into 1/4-inch dice (3/4 cup). Steam potato in a steamer set over boiling water, covered, until just cooked through, about 3 minutes. Transfer to a plate to cool.
- Cook onion in butter in a 10-inch heavy skillet over moderately low heat, stirring, until softened, 8 to 10 minutes. Transfer to plate to cool.
- Whisk together 2 eggs, cream, thyme, salt, and pepper in a large bowl until combined well. Stir in potato, onion, and cheese.
- Whisk remaining egg in a small bowl and reserve for egg glaze.
- Put oven rack in middle position and put a large baking sheet on it, then preheat oven to 425°F.
- Roll out larger piece of dough into a 13-inch round on a lightly floured surface with a lightly floured rolling pin and fit into tart pan. Spoon filling evenly into pie shell and brush some of egg glaze on rim of shell.
- Roll out smaller piece of dough into a 12-inch round in same manner, then cover pie with it. Trim edges of pie, leaving a 1/2-inch overhang, then press edges together and crimp decoratively.
- Brush top of pie lightly with some of remaining egg glaze. Cut several steam vents in top crust with a small sharp knife.
- Bake pie on hot baking sheet until golden brown, 35 to 40 minutes. Cool on a rack at least 15 minutes. Serve warm or at room temperature.