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1
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.
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2
Drizzle evenly with 8 tablespoons (1/2 cup) ice water and gently stir with a fork (or pulse in processor) until incorporated.
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3
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.)
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4
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.
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5
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.
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6
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.
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7
Whisk together 2 eggs, cream, thyme, salt, and pepper in a large bowl until combined well. Stir in potato, onion, and cheese.
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8
Whisk remaining egg in a small bowl and reserve for egg glaze.
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9
Put oven rack in middle position and put a large baking sheet on it, then preheat oven to 425u00b0F.
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10
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.
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11
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.
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12
Brush top of pie lightly with some of remaining egg glaze. Cut several steam vents in top crust with a small sharp knife.
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13
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.
