Start the recipe by preparing the pasta. Fill half of a large pot with lightly salted water and bring it to a boil over high heat. Reduce the heat to medium-high, add the manicotti shells to the boiling water, and cook for 8 to 10 minutes or until al dente. Pour the cooked manicotti shells into a colander to drain the hot water, rinse them with cold water, then drain them and set them aside.
Take a medium sized mixing bowl and combine in it all the three cheeses (ricotta cheese, mozzarella cheese and Parmesan cheese), preserving ½ cup of mozzarella cheese and ¼ cup of Parmesan cheese.
Then, add one slightly beaten egg and one tablespoon of finely chopped parsley to the bowl and mix.
Season the cheese and egg mixture with salt, pepper and nutmeg powder, and mix it well.
Spoon two heaping tablespoons of this cheese mixture into each manicotti shell, and set the prepared shells aside.
Take a 13x9 inch baking pan, grease it with butter or vegetable shortening, and cover bottom of the pan with a thin layer of spaghetti sauce.
Arrange the stuffed manicotti shells in the baking pan in a single layer and pour the remaining spaghetti sauce over them.
Sprinkle with the remaining mozzarella and Parmesan cheese, and bake it covered for 15 minutes in an oven preheated at 350 degrees Fahrenheit.
After 15 minutes, uncover the shells and bake for another 15 to 20 minutes, or until the cheese topping is bubbly and golden brown.
Remove from the oven, and serve hot with a salad.