Chop the onion and garlic and fry briefly in a little oil. Add the frozen spinach with a small splash of water and allow to thaw. Sauté briefly and season with salt, pepper and nutmeg.
Spread the finished spinach in a baking dish and pour the tortellini over it. Be sure to use the fresh, chilled tortellini, which only need to cook for 2 - 3 minutes, otherwise they won`t be cooked in the casserole. Pour the tomato sauce over the tortellini and distribute it so that everything is covered. Here you can either make a tomato sauce yourself beforehand or take a seasoned one out of the glass. Finally, tear the mozzarella into small flakes and cover the casserole with it. Alternatively, simply use the cheese of your choice.
Bake at 200 degrees Celsius for about 20 minutes until the cheese is lightly browned.