Step 1: In a small bowl, combine the breadcrumbs and milk. Let the mixture sit for 5-10 minutes until a thick paste (panade) forms.
Step 2: In a large mixing bowl, combine the ground beef, ground pork, Parmesan cheese, parsley, minced garlic, salt, pepper, and the beaten egg.
Step 3: Add the milk-soaked breadcrumbs to the meat mixture. Using your hands, gently mix until the ingredients are just combined, being careful not to overwork the meat.
Step 4: Scoop about 2 tablespoons of the mixture and roll between your palms to form a ball about 1.5 inches in diameter. Repeat until all meat is used (makes about 18-20 meatballs).
Step 5: Heat olive oil in a large skillet over medium heat. Add meatballs in batches, making sure not to crowd the pan.
Step 6: Brown the meatballs on all sides for about 8-10 minutes. They do not need to be fully cooked through if you plan to simmer them in sauce.
Step 7: Transfer the browned meatballs into a simmering pot of marinara sauce and cook for an additional 20 minutes, or finish cooking them in the skillet until the internal temperature reaches 165°F.