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. They don’t have to be perfect.

To cook on the stovetop: Pour enough oil into a large skillet to coat the bottom and place over medium-high heat. Add meatballs and brown on all sides. Cover and continue cooking until firm and cooked through, about 15 minutes.

To finish cooking in tomato sauce: Transfer meatballs to a pot of simmering sauce just after browning. Simmer until cooked through, 10-12 minutes.

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