Defrost the spinach. Chop the onion and garlic and sauté in a saucepan. Then add the spinach and cook for 10-15 minutes. Season the spinach with salt, pepper and nutmeg. Dab the fish, season with lemon, salt and pepper and place in a baking dish. Put the spinach on the fish, then add the cheese and bake in the preheated oven at 180 ° for about 25 minutes.