Step 1

In a large pot, combine the whole tomatoes and their juices, crushing the tomatoes with your hands as you add them, tomato sauce, onions, and butter. Bring to a boil over medium-high heat, then reduce the heat to a simmer. Cook, stirring often, until the onions are softened, about 40 minutes. Using tongs, carefully discard the onions. See Note.

Step 2

While the sauce is simmering, make the meatballs. In a large bowl, combine the beef, pork, eggs, breadcrumbs, parsley, Parmesan, garlic, salt, and pepper. Mix well, then with wet hands, form golf ball–size balls (about 2 tablespoons each). You should have 40 to 45 meatballs.

Step 3

Carefully add the meatballs to the sauce. Return the sauce to a simmer over medium heat, then reduce the heat to low, cover, and cook for 30 minutes, or until the meatballs are cooked through.

Step 4

Serve the meatballs and tomato sauce over spaghetti topped with additional parsley and Parmesan.