(vegetarian) Mushroom Lasagna

16 ounce box lasagna noodles
3 tablespoons olive oil
1 pound of mushrooms
1 1/2 teaspoon salt
One medium onion, chopped fine
Four cloves garlic, chopped fine
6 tablespoons of butter
4 tablespoons of flour
4 cups cups of milk
1/4 teaspoon nutmeg
1/4 teaspoon black pepper
4 ounces crème fraîche
One cup Parmesan
3 cups mozzarella
15 ounces whole milk, ricotta
Set oven to 400°