Prepare the Breadcrumb Mixture: Place the breadcrumbs in a small bowl and pour milk over them, allowing them to soak for 5 minutes. Add egg yolks, cheese, garlic, salt, and black pepper, then mash to form a coarse paste.
Mix the Meatball Ingredients: In a large bowl, combine the ground beef, pork, and grated onion (if using). Add the breadcrumb mixture and parsley, mixing thoroughly with hands, a large fork, or a wooden spoon until well-blended. Alternatively, use a stand mixer on low speed. This mixture can be prepared up to two days in advance and stored in the refrigerator.
Shape the Meatballs: Using a 1/4-cup measure or a cookie scoop, portion out the mixture and shape into meatballs. Avoid overworking to maintain a tender texture.
Stovetop Cooking Method: Pour enough olive oil to coat the bottom of a large skillet and place over medium-high heat. When hot, add the meatballs, ensuring they sizzle upon contact. Brown on all sides, then cover, reduce heat, and cook until an instant-read thermometer registers 165°F, about 15 minutes total. Repeat as necessary, refreshing oil with each batch.
Cooking in Tomato Sauce: For a saucy finish, transfer browned meatballs into a pot of simmering tomato sauce. Cook gently for 10–12 minutes until thoroughly heated and infused with sauce.
Baking Method: Preheat oven to 425°F. Arrange meatballs on a rimmed baking sheet lightly coated with olive oil. Bake for 10 minutes, then turn the meatballs and bake for an additional 10–12 minutes until golden brown and cooked through.
Serve: Serve warm with marinara or your choice of sauce.