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1
Stir together yeast, milk, and sugar in bowl of a stand mixer fitted with paddle attachment (or in a bowl to mix by hand) and let stand until foamy, about 5 minutes. (If mixture doesn't foam, start over with new yeast.)
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2
Add flour, salt, egg, and butter and mix at low speed (or stir) until a dough forms. Increase speed to medium-high and beat 3 minutes (or knead by hand 5 minutes).
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3
Sprinkle dough lightly with flour and cover bowl with a kitchen towel (not terry cloth). Let dough rise in a draft-free place at warm room temperature until doubled, 1 1/2 to 2 hours.
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4
Cook onions and bacon in butter with 1 teaspoon salt and 1/2 teaspoon pepper in a 12-inch heavy skillet, covered, over medium-high heat, stirring occasionally, until onions are softened, about 20 minutes.
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5
Remove lid and cook, stirring occasionally, until onions are golden, 20 to 30 minutes. Cool onions.
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6
Whisk together sour cream and yolks, then stir into onions.
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7
Preheat oven to 375u00b0F with rack in middle.
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8
Roll out dough on a lightly floured surface with a lightly floured rolling pin into a 12-inch round. Fit dough into springform pan (stretching slightly if necessary to hang over edge) and spread filling evenly in it. Fold edges of dough over filling, leaving some filling exposed in center. Let pie stand 20 minutes at room temperature.
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9
Bake until crust is golden-brown and filling is bubbling, about 11/4 hours. Cool slightly before serving.
