Combine flour, cold butter, 1 egg, and salt in a bowl.

Knead together, adding water in small amounts until a smooth dough forms.

Shape dough into a ball, flatten, and wrap in plastic wrap.

Refrigerate for at least 30 minutes.

Preheat oven to 400 degrees F (200 degrees C).

Grease a 10-inch springform pan.

Heat olive oil in a skillet over medium heat.

Cook sliced leeks until translucent, about 5 minutes.

Remove dough from the fridge.

Dust a work surface with flour.

Roll out dough into a circle slightly larger than the springform pan.

Fold dough carefully in half, lift, and unfold over the pan.

Press dough over the bottom and 1 inch up the sides.

Whisk 4 eggs and creme fraiche in a bowl.

Stir in cooked leeks, grated Gruyere cheese, grated Emmentaler cheese, and cubed ham.

Season with white pepper and nutmeg.

Pour the filling over the crust and spread evenly.

Bake in the preheated oven until the filling is set, about 30 minutes.