In a large mixing bowl, mix together all the meatball ingredients together into a smooth paste. (Your bare hands are the best tools for this job.) To eliminate any air holes in the mixture, smack it down into the bowl with some force, repeating a few times. Take a piece of the mixture, about the size of a walnut, and form it into a smooth ball by rolling it between the palms of your hands in a circular motion.
Fry the meatballs in the olive oil (or, for extra savor in the Neapolitan style, lard) until they are nice and brown on all sides.
Add the chopped onion and garlic and sauté over gentle heat for a few minutes, until the onions are soft. Add the tomatoes and simmer for about 15-20 minutes, until the tomato and fat begin to separate.
Cook the spaghetti al dente and drain well. Dress lightly with the tomato sauce. Plate the spaghetti and top with some more sauce and 2-3 meatballs. Top with freshly grated Parmesan or pecorino romano cheese, if you like, and enjoy!