Crust:

1. In a food processor, pulse together the flour and salt.
2. Add butter and pulse until the butter is the size of peas.
3. In a bowl, mix 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.
4. Add remaining sour cream mixture and pulse again. If the dough is crumbly or seems dry, add 1 or 2 tablespoons more ice water and pulse until the dough comes together into large lumps and all the dry flour is incorporated.
5. 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.
6. Line a deep 9-inch diameter cake pan 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.
7. Roll out the dough on a generously floured surface and place the dough into the prepared pan, pressing the dough into the bottom of the pan.
8. Trim off any dough that hangs over the edge of the pan.
9. Refrigerate dough-lined pan for 30 minutes, then freeze for 20 minutes.
10. Preheat oven to 375 degrees F with the rack set in the lower-middle position.
11. Line dough with aluminum foil and fill completely with pie weights, dried beans, or pennies.
12. Bake 30-40 minutes, or until the edges of the crust begin to brown and the bottom of the crust is still light. Carefully remove the foil and pie weights and return the pan to the oven, baking until golden brown, about 15 to 20 minutes longer.
13. Begin to prepare the filling. Remove shell from oven and brush immediately with egg white. Set aside while you prepare the filling.

Filling:

1. Reduce oven temperature to 350 degrees F.
2. Place 2 tablespoons of butter into a pan, heat and add sliced leeks. Cook, stirring frequently, until softened and browned slightly about 10 minutes. Set aside and allow to cool.
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 salmon in the bottom of the crust. Gently pour the egg mixture over the top.
5. Bake until the top of the quiche is golden and a toothpick inserted in the center comes out clean, 1 ¼ to 1 ½ hours. Transfer to a wire rack to cool at least 30 minutes.