Prepare the Crust:

1. In the bowl of a food processor, pulse together the flour and salt. Add butter and pulse until the butter is the size of peas.
2. In a small bowl, combine the sour cream and 1/4 cup of ice water. Add half of the sour cream mixture to the food processor and pulse a few times. Add remaining sour cream mixture and pulse again.
3. Pinch the dough between your fingers. If dough is crumbly and seems too dry, add 1 or 2 tablespoons more ice water and pulse until dough comes together into large lumps and all the dry flour is incorporated.
4. Form the dough into a large flat disk, about 6 inches in diameter, and wrap in plastic wrap. Refrigerate until firm, about 1 to 2 hours.
5. Line a deep 9-inch diameter cake pan or spring-form pan with 2 long pieces of aluminum foil, allowing a few inches of foil to hang off the sides of the pan.Lightly spray the foil with cooking spray.
6. Roll out the dough on a generously floured surface and gently place the dough into the prepared pan, pressing the dough into the bottom of the pan. Trim off any dough that hangs over the edge of the pan more than 1 inch.
7. Refrigerate dough-lined pan for 30 minutes, then freeze for 20 minutes. (Make Ahead: If you are planning to bake the quiche the next day, refrigerate the dough overnight at this point, and then continue with the remaining steps in the morning.)
8. Preheat oven to 375 degrees F with the rack set in the lower-middle position.
9. Line dough with aluminum foil and fill completely with pie weights. (If you don't have enough pie weights, you can use dried beans, or I have even used spare change before.)
10. Bake 30-40 minutes, or until the edges of the crust are beginning to brown but the bottom of the crust is still light in color. Carefully remove the foil and pie weights and return the pan to the oven, baking until golden brown, about 15 to 20 minutes longer. (Now is a good time to start preparing the filling.)
11. Remove shell from oven and brush immediately with egg white. Set aside while you prepare the filling.

For the Filling:

1. Reduce oven temperature to 350 degrees F.
2. Cook bacon in a skillet set over medium heat until crisp. Transfer to a paper towel-lined plate and set aside. Discard all but 2 tablespoons of fat from the pan. Return to medium heat and add chopped onion. Cook, stirring frequently, until softened and browned slightly, about 10 minutes. Set aside and allow to cool a bit.
3. In a large bowl, whisk together the cornstarch and 3 tablespoons of the milk. Whisk in the remaining milk, eggs, yolk, heavy cream, salt, pepper, nutmeg, and cayenne. Whisk until smooth.
4. Sprinkle the cheese, onions, and bacon in the bottom of the crust. Gently pour the egg mixture over the top.
5. Bake until top of quiche is golden and a toothpick inserted in the center comes out clean, 1 1/4-1 12 hours. Transfer to a wire rack to cool at least 30 minutes. (Do not serve immediately after baking as the quiche needs time to cool and set or your slices won't come out clean.)
6. Enjoy!