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1
Whisk together flour, sugar, and salt in a large bowl (or pulse in a food processor). Blend in butter with your fingertips or a pastry blender (or pulse in processor) until mixture resembles coarse meal with some roughly pea-size butter lumps. Beat together egg and 3 tablespoons ice water with a fork and stir into flour mixture (or pulse) until incorporated.
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2
Squeeze a small handful: If dough doesn't hold together, add more ice water, 1/2 tablespoon at a time, stirring (or pulsing) until incorporated. (Do not overwork mixture or pastry will be tough.)
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3
Gather dough together, with a pastry scraper if you have one, and press into a 5-inch disk. Chill, wrapped in plastic wrap, until firm, at least 1 hour.
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4
Roll out dough between 2 sheets of plastic wrap into a 12-inch round. (If dough becomes too soft to work with, chill until firm, 10 to 15 minutes.) Fit dough into tart pan (discard plastic wrap), pressing against side. Run rolling pin over top edge to cut off excess dough, then press side to make dough come up slightly above edge of pan. Chill until firm, about 30 minutes.
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5
Line a 4-sided sheet pan with foil and put in middle of oven. Preheat oven to 425u00b0F.
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6
Lightly toast walnuts in sheet pan, about 5 minutes. Transfer to a bowl and return pan to oven.
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7
Heat cream in a small saucepan over medium-low heat.
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8
Bring water and 1 1/2 cups sugar to a boil in a 3-quart heavy saucepan over medium heat, stirring until sugar has dissolved, then wash down any sugar crystals from side of pan with a pastry brush dipped in cold water. Boil, without stirring, swirling pan occasionally so caramel colors evenly, until amber.
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9
Remove from heat and immediately but slowly add hot cream (it will bubble furiously). Once bubbles subside, stir in walnuts and butter and cook over medium heat, stirring, 2 minutes.
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10
Pour filling into tart shell and sprinkle evenly with remaining 2 tablespoons sugar. Bake in sheet pan 25 minutes (check tart after 15 minutes, and if top becomes too dark in spots, loosely cover with foil).
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11
Reduce oven to 325u00b0F and bake until filling is set and deep golden-brown and crust is golden, 10 to 15 minutes more (filling may spill over edge in spots).
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12
Cool tart to warm in tart pan, 30 to 45 minutes, then remove side of pan (do not cool completely before removing side of pan or tart will stick). Serve warm or at room temperature.
