Put the breadcrumbs in a small bowl and pour the milk over. Let them soak for 5 minutes. Add the egg yolks, cheese, garlic, salt, and pepper to the bowl and mash together to form a coarse paste.

Put the beef, pork, and onion (if using) in a large bowl and mix with a fork to blend. Add the bread mixture and parsley. Blend everything together well, using your hands, large wooden spoon or large fork.

Use a ¼-cup measuring cup or cookie scoop to portion the mixture and roll lightly into balls.

To cook on the stovetop, pour enough oil into a large skillet to coat the bottom and place over medium-high heat. When the pan is hot, add as many meatballs as will fit without crowding. Brown the meatballs on all sides, cover the pan, lower the heat and continue cooking until the meatballs are firm and cooked through, about 15 minutes total.

To finish cooking in tomato sauce, transfer the browned meatballs to a pot of simmering sauce and simmer for 10-12 minutes until cooked through.

To bake the meatballs, preheat the oven to 425 degrees. Arrange the meatballs on an olive-oiled coated baking sheet. Bake for 10 minutes, then turn the meatballs over and continue baking until browned and cooked through, another 10-12 minutes.

Serve the warm meatballs with marinara sauce or your favorite prepared tomato sauce.