• 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. The meatball mixture can be mixed and refrigerated 2 days ahead of shaping and cooking.
• Form the meatballs: 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 — craggy meatballs have character and hold the sauce better!

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. Note: They should sizzle as soon as they hit they pan or the pan isn’t hot enough.
• Brown the meatballs on all sides. Cover the pan, lower the heat and continue cooking and until the meatballs are firm and cooked through (165 degrees on a instant thermometer), which should take about 15 minutes total. Repeat the frying with remaining meatballs, pouring out the oil and adding a fresh layer each time.

  To finish cooking in tomato sauce:

  If you'd rather finish cooking the meatballs in tomato sauce (my preference), transfer them to a pot of simmering sauce just after browning on all sides. They won't be cooked all the way through. 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 with a spatula or pair of tongs. 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.