Step 1

FOR THE CRUST: Combine ice water and sour cream in bowl. Process flour, sugar, and salt in food processor until combined, about 5 seconds. Scatter butter over top and pulse until butter is size of large peas, about 10 pulses. Add sour cream mixture and pulse until dough forms clumps and no dry flour remains, about 12 pulses, scraping down sides of bowl as needed.

Step 2

Turn out dough onto sheet of plastic wrap and form into 4-inch disk. Wrap tightly in plastic and refrigerate for 1 hour. (Wrapped dough can be refrigerated for up to 2 days or frozen for up to 1 month. If frozen, let dough thaw completely on counter before rolling.)

Step 3

Adjust oven rack to middle position and heat oven to 350 degrees. Let chilled dough sit on counter to soften slightly, about 10 minutes, before rolling. Roll dough into 12-inch circle on lightly floured counter. Loosely roll dough around rolling pin and gently unroll it onto 9-inch pie plate, letting excess dough hang over edge. Ease dough into plate by gently lifting edge of dough with your hand while pressing into plate bottom with your other hand.

Step 4

Trim overhang to ½ inch beyond lip of plate. Tuck overhang under itself; folded edge should be flush with edge of plate. Crimp dough evenly around edge of plate using your fingers. Push protruding crimped edge so it slightly overhangs edge of plate. Wrap dough-lined plate loosely in plastic and freeze until dough is firm, about 15 minutes.

Step 5

Place chilled pie shell on rimmed baking sheet. Line with double layer of parchment paper, covering edges to prevent burning, and fill with pie weights. Bake until edges are light golden brown, about 20 minutes. Remove parchment and weights, rotate plate, and bake until crust bottom dries out and turns light golden brown, 15 to 20 minutes. If crust begins to puff, pierce gently with tip of paring knife. Set aside. (Crust needn't cool completely before adding filling.)

Step 6

FOR THE FILLING: Meanwhile, cook bacon in 12-inch nonstick skillet over medium heat until crispy, 5 to 7 minutes. Using slotted spoon, transfer bacon to paper towel–lined plate. Pour off all but 1 tablespoon fat from skillet. Add onion to skillet and cook over medium heat, stirring frequently, until softened and lightly browned, 8 to 10 minutes. Set aside to cool slightly.

Step 7

Whisk ¼ cup cream and cornstarch in large bowl until cornstarch dissolves. Whisk in eggs, salt, pepper, and remaining 1 cup cream until mixture is smooth.

Step 8

Scatter bacon, onion, and Gruyère evenly over crust. Pour custard mixture over top. Tent quiche with lightly greased aluminum foil. Bake on baking sheet until toothpick inserted in center of quiche comes out clean and center registers 170 degrees, 50 minutes to 1 hour. Transfer to wire rack, discard foil, and let rest until cool to touch, about 2 hours. Slice and serve. (Quiche can be refrigerated for up to 3 days.)