Preheat the oven to 425°F. Line a rimmed baking sheet and lightly oil it.
Soak the breadcrumbs in the milk in a large bowl for 5 minutes.
Whisk in the beaten egg, Parmesan, onion, garlic, parsley, Italian seasoning, salt, and pepper.
Add the beef and pork. Gently mix with your hands until just combined; do not overwork.
Form golf ball–size meatballs and arrange on the prepared sheet. Drizzle or brush with olive oil.
Bake until lightly browned, 12–15 minutes.
Warm the marinara in a large pot over medium heat. Transfer meatballs to the sauce and simmer 10–15 minutes, until cooked through.
Serve hot with extra sauce over pasta, polenta, or in toasted sub rolls.