In a large mixing bowl, combine ground beef, Italian sausage, minced garlic, chopped parsley, parmesan cheese, salt, pepper, bread crumbs, eggs, and milk. Mix until well combined.
Roll the mixture into 4-5 ounce meatballs using a medium-sized ice cream scooper for even portions.
Preheat your oven to 425°F (220°C).
Heat 1 tablespoon of olive oil in a pan over medium heat. Brown the meatballs on both sides for about 2-3 minutes until a crispy sear forms. Cook them halfway through, as they will finish cooking in the oven.
In a casserole dish, spread a few tablespoons of marinara sauce on the bottom to prevent sticking. Place the meatballs in the dish and pour the remaining marinara sauce over them. Top with shredded mozzarella cheese.
Cover the casserole dish with foil and bake at 425°F (220°C) for 25 minutes, or until the cheese is fully melted and the meatballs are cooked through.
Serve hot, garnished with fresh parsley.