In a large mixing bowl, combine ground beef, Italian sausage, garlic, parsley, parmesan cheese, salt, pepper, bread crumbs, eggs, and milk. Mix until everything is fully infused together.
Roll into 4-5 ounce (113-142 g) meatballs. For even amounts, use a medium-sized ice cream scooper.
Over medium heat, add 1 tablespoon of olive oil to a pan, and brown the meatballs on both sides for about 2-3 minutes or until you create a nice crispy sear. Cook about half way through. They will finish cooking in the oven.
In a casserole dish, spread a few tablespoons of marinara sauce to the bottom of the pan to help prevent sticking. Then place meatballs in the casserole dish and add the remaining jar of marinara sauce. Top with 2 cups of shredded mozzarella cheese.
Cover in foil and bake at 425°F (220°C) for 25 minutes or until the cheese is fully melted and the meatballs are fully cooked.