Bring milk to boil in a small saucepan, set aside. Melt four tablespoons butter in a large saucepot over medium heat. Add flour and stir to combine, cooking for two minutes. Remove from heat and slowly add milk, stirring constantly. Return to heat and bring to a boil once more. Reduce heat to low and season with half a teaspoon salt and pepper. Cook until sauce is thick enough to coat the back of a spoon.

Remove from heat and stir in six ounces of the grated cheese, approximately one cup. Cover the surface of the sauce with plastic wrap to avoid a skin on the sauce and set aside.

Defrost spinach and place in a strainer or colander. Press and drain as much liquid as possible.

Heat two tablespoons butter in a large skillet. Add half the minced onion. Cook two minutes or until translucent. Add spinach, season lightly with salt and pepper, and sauté until most of the moisture has evaporated. Add one cup of the cheese sauce to the spinach mixture and stir to combine. Remove from heat and set aside.

Combine cottage cheese and egg and remaining salt. Heat two tablespoons butter in a skillet, add remaining onions and sauté two minutes, or until translucent. Add mushrooms and continue to sauté until soft and excess moisture has cooked away. Remove from heat; add cottage cheese mixture and one cup cheese sauce. Combine well.

Butter a nine-inch round baking dish. Place one crepe in the dish. Spread with two tablespoons spinach mixture. Top with a crepe. Spread with two tablespoons of mushroom mixture. Top with another crepe. Continue alternating crepes with layers of spinach and mushroom fillings, ending with a crepe. (Up until this point, the gateau may be prepared in advance and frozen.)

Heat oven to 350 degrees Fahrenheit.

Spread top of gateau with remaining cheese sauce and sprinkle with remaining grated Swiss cheese. Bake 30 to 40 minutes or until cheese is lightly browned. Serve hot, sliced in wedges.