Preheat the oven to 180 C. In a saucepan put salted water to a boil and add the spinach. It is better to stand 3-4 minutes in boiling water. Strain it through a fine strainer, press with a large spoon to get out as much water. Cut it into small pieces. In a large skillet melt 1 tbsp butter on medium intensity stove. Add onion and garlic, stirring for 1 minute, add the mushrooms, salt, pepper and stir until the mushrooms are soft and water evaporates them, about 5 minutes. Move from heat to cool. TOPPING: In a large saucepan, melt the remaining 4 tbsp butter on medium heat, add the flour, stir until golden, about 2 minutes. With stirring, slowly add the milk, continue stirring until thickened. Season to taste with salt, pepper, nutmeg and 1 cup Parmesan, stir for about 2 minutes, remove from the heat. In bowl, combine cheese, mozzarella and Rikotata. Add 1 tbsp of sauce topping them. In the pan put four sheets to each other, and all of them put the white sauce, mushrooms and spinach. Place crust on each new crust. Again alternate white sauce, mushrooms, spinach, this time put the mixture and cheese. So as alternate endings must have a crust. Pour on top with the remaining white sauce and sprinkle with Parmesan. Optional you can add basil. Put foil and bake for 10 minutes, then remove it and finish cooking until golden brown catch.