Preheat your oven to 350F and spray a 9x9 casserole dish with cooking spray.
Combine ground beef, sausage, parmesan, parsley, olive oil, eggs, garlic, salt and pepper in a large mixing bowl. Careful not to overwork the meat.
Roll your mixture into 12 -13 meatballs.
Spray avocado oil in a large skillet and fry each meatball over medium heat until brown on all sides. Use a non stick skillet with the spray oil for even more assurance that your meatballs do not stick.
Place your meatballs inside a 9x9 casserole dish. Add the jar of marinara sauce around the meatballs
Sprinkle the shredded mozzarella cheese evenly over the top of the meatballs.
Bake in the oven for 20 minutes or until an internal meat thermometer reads 165F in the center of the meatballs. If you would like your cheese to get a little more brown, you can set your oven to broil for a few minutes extra.