Preheat the oven to 400°F. Combine the breadcrumbs and whole milk in a small bowl and let sit 5 minutes until soft and pasty.
In a large bowl, combine the ground beef, ground pork, panade, egg, parmesan, garlic, parsley, salt, and pepper. Mix by hand just until evenly combined.
Divide the mixture into 6 equal portions and roll each gently into a ball.
Place the meatballs on a rimmed baking sheet, spaced apart, and bake for 20–22 minutes until light golden on the outside.
Transfer the meatballs to an oven-safe skillet or baking dish. Pour the marinara sauce over and around them, then top each with a slice of mozzarella.
Bake at 400°F for another 8–10 minutes, until the sauce is bubbling and the cheese is fully melted.
Switch to broil for the final 2 minutes to lightly brown the mozzarella, watching closely to prevent burning.
Rest for 3 minutes before serving.