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. You can also mix in a standing mixer on low speed, just until blended.

Form the meatballs: 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 in the pan 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 (165 degrees on an instant thermometer), which should take about 15 minutes total.

To finish cooking in tomato sauce: Transfer the meatballs to a pot of simmering sauce just after browning on all sides. Simmer until they're cooked through, 10-12 minutes.

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

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