First prepare the pasta according to the instructions on the packet.
Meanwhile, season the milk with nutmeg and warm it up, but do not boil it. Melt the butter in a saucepan over low heat and stir in the flour. Simmer gently and add the milk in small portions and stir. Then simmer until the sauce is thick and then add the cheddar and three quarters of the parmesan and stir until the cheese has melted. Now chop the spinach and add it to the sauce. Season to taste with salt and pepper.
Mix the sauce with the drained pasta, place in a baking dish and sprinkle with the rest of the parmesan. Place the mold under the grill (highest setting) and wait until the cheese has melted and turned slightly brown.