Stir together the flour, salt, and butter until crumbly in a mixer or food processor. Add the egg yolk and ice waterand blend until it balls up. Break off pieces and press it into your pie pan or quiche dish. Prick with a fork a few times and bake at 350 for 5 minutes to partially bake it.
Slice the onions and sautee them in the butter until soft. Set aside.
Mix together the leftover egg white and 3 eggs, milk, nutmeg and cayenne pepper. Put the onions in the pie shell, sprinkled the cheese over them and pour the custard mix in it.
Baked at 450 for 15 minutes, then at 350 for 30 more.