Pull the butter out of the fridge 30 minutes prior to starting the dough to allow it to soften. Once the butter is softened, put the flour, salt, and butter in a large bowl and incorporate the butter until you have a sandy consistency. Add the egg yolk and water and knead for a few minutes until the dough forms a smooth ball.
Preheat oven to 425 degrees. Let the dough rest in the fridge for 15 minutes before rolling it out and shaping into a circle. Place it in a pie dish and crimp the edges. Prebake it in the oven for 5 minutes.
Filling
In a large pan on medium-high heat, saute leeks and onion for about 5 minutes before adding the garlic and letting it cook for another 5 minutes or until the leeks and onions are nice and soft. Then, add 2 tbsp crème fraiche, salt, pepper, and nutmeg and mix well to incorporate. Pour this mixture into the pre-cooked pie crust. Layer over half of the salmon slices and set aside.
In a separate bowl, mix together the eggs, milk, cheese, salt, pepper, and another 2 tbsp crème fraiche. Pour this egg mixture on top of the leek, onion, garlic and salmon base, and top with the rest of the salmon slices. Sprinkle over a bit more cheese and bake for 30 minutes.