In a large skillet, melt the butter over medium heat. Add the onion, salt, pepper, and nutmeg and cook until the onions very soft but not browned about 5 minutes. Add the remaining ¾ cup of milk and bring to a boil. Stir in the cornstarch mixture and boil until thickened, just a few seconds. Stir in ¼ cup of parmesan cheese and the cheddar cheese. Lower heat and stir until the cheeses are melted. Add the cooked pasta and stir to coat the pasta with the cheese sauce. Taste and add some more salt and pepper if needed.