Preheat your oven to 375°F. In a large mixing bowl, combine the ground beef, Italian sausage, ricotta cheese, and the large egg. Add the minced garlic, Italian seasoning, salt, and black pepper. Mix until well combined and form into meatballs about 1 to 1.5 inches in diameter. Place in a greased baking dish.
Pour the marinara sauce evenly over the meatballs in the baking dish.
Sprinkle the shredded mozzarella cheese and grated Parmesan cheese over the top of the meatballs and sauce.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 15-20 minutes until the cheese is melted and bubbly.
Let the casserole cool for a few minutes before serving. Garnish with fresh basil if desired.