FOR THE DOUGH:
Process flour and salt together in food processor until combined, about 3 seconds. Add butter and pulse until butter is size of large peas, about ten 1-second pulses.
Mix sour cream and ¼ cup ice water in small bowl until combined. Add half sour cream mixture to flour mixture; pulse for three 1-second pulses. Repeat with remaining sour cream mixture. Pinch dough with fingers; if dough is floury, dry, and does not hold together, add 1 to 2 tablespoons ice water and process until dough forms large clumps and no dry flour remains, three to five 1-second pulses.
Turn dough out onto work surface and flatten into 6-inch disk; cover disk in plastic wrap and refrigerate until firm but not hard, 1 to 2 hours, before rolling. (Dough can be refrigerated for up to 24 hours. Let thoroughly chilled dough stand at room temperature 15 minutes before rolling.)
Cut two 16-inch lengths of foil. Arrange foil pieces in round 9 by 2-inch cake pan so they are perpendicular, pushing them into corners and up sides of pan; press overhang against outside of pan. Spray foil lightly with nonstick cooking spray.
Roll out disk of dough on generously floured work surface to 15-inch circle (about ¼ inch thick). Roll dough loosely around rolling pin and unroll into cake pan. Working around circumference, ease dough into pan by gently lifting edge of dough with 1 hand while pressing into pan bottom with other. Trim any dough that extends more than 1 inch over edge of pan. Patch any cracks or holes with dough scraps as needed. Refrigerate any remaining dough scraps. Refrigerate dough-lined pan until firm, about 30 minutes, and then freeze for 20 minutes.
Adjust oven rack to lower-middle position and heat oven to 375°F. Line dough with parchment or foil and fill completely with pie weights or dried beans, gently pressing weights into corners of shell. Bake on rimmed baking sheet until exposed edges of dough are beginning to brown but bottom is still light in color, 30 to 40 minutes. Carefully remove parchment and pie weights. If any new holes or cracks have formed in dough, patch with reserved scraps. Return shell to oven and bake until bottom is golden brown, 15 to 20 minutes longer. Remove shell from oven and brush interior with egg white. Set aside while preparing filling. Reduce oven temperature to 350 degrees.
FOR THE CUSTARD:
Cook bacon in 12-inch skillet over medium heat until crisp, 8 to 12 minutes. Transfer to paper towel-lined plate and discard all but 2 tablespoons bacon fat from skillet. Return to medium heat, add onions, and cook, stirring frequently, until softened and lightly browned, about 12 minutes. Set onions aside to cool slightly.
Place cornstarch in large bowl; add 3 tablespoons milk and whisk to dissolve cornstarch. Whisk in remaining milk, eggs, yolk, cream, salt, pepper, nutmeg, and cayenne until mixture is smooth.
Scatter onions, bacon, and Gruyère evenly over crust. Gently pour custard mixture over filling. Using fork, push filling ingredients down into custard and drag gently through custard to dislodge air bubbles. Gently tap pan on countertop to dislodge any remaining air bubbles.
Bake until top of quiche is lightly browned, toothpick inserted into center comes out clean, and center registers 170°F on instant-read thermometer, 1¼ to 1½ hours. Transfer to wire rack and let stand until cool to touch, about 2 hours.
When ready to serve, use sharp paring knife to remove any crust that extends beyond edge of pan. Lift foil overhang from sides of pan and remove quiche from pan; gently slide thin-bladed spatula between quiche and foil to loosen, then slide quiche onto serving plate. Cut into wedges and serve.
SWAPPING A PIE PLATE FOR A CAKE PAN:
Unlike a pie plate, a deep cake pan requires a foil sling (fashioned from two lengths of foil) to remove the quiche.
Instead of a 12-inch round, roll out dough to a 15-inch circle big enough to fit in the cake pan with plenty of overhang.
Roll the dough loosely around a rolling pin and unroll it into the foil-lined cake pan.
Gently ease the dough into the pan. Trim, leaving a generous 1-inch overhang to anchor the dough.
NOTE:
To prevent the crust from sagging during blind baking, make sure it overhangs the pan’s edge and use plenty of pie weights (about 3 to 4 cups). The cooled quiche can be served warm or at room temperature, or refrigerated for up to 3 days and reheated. To reheat the whole quiche, place it on a rimmed baking sheet on the middle rack of a 325-degree oven for 20 minutes. Reheat slices at 375°F for 10 minutes.
