Throw the pasta in boiling salted water and cook according to the instructions on the package. Meanwhile, chop the onion and brown it in a pan with a tablespoon of oil over low heat. Add the spinach and cook over medium heat, stirring occasionally until it is completely defrosted. Add the Parmesan, salt, pepper and nutmeg to taste and blend with a minipimer until a smooth cream is obtained. Leave it to cool and cut the mozzarella di bufala into small cubes. When the pasta is cooked, drain it and pour it into the pan with the cream of spinach. Mix well, add the mozzarella di bufala keeping some small cubes aside. Pour the pasta seasoned with the spinach sauce in a previously greased baking dish with a tablespoon of oil and decorate with the remaining mozzarella cubes. Sprinkle with another grated parmesan and bake in preheated oven at 200 ° C for about twenty minutes or until the mozzarella has melted.