Preheat oven to 425° Fahrenheit.
In large bowl, add all meatballs ingredients: ground beef and sausage, minced garlic, egg, milk, breadcrumbs, Italian seasoning, salt and pepper. Mix together until well combined but without over-mixing.
Scoop 2 tablespoons of meat mixture and roll between palm of hands, forming meatballs of even size. Repeat process with remaining meatball mixture until all gone, then set meatballs aside in plate or in baking sheet.
Bring large pot of water to boil, and cook pasta following package directions for al-dente (see Notes). Once cooked, drain pasta into colander, then add to 9x13 inch baking dish.
Pour jar of marinara sauce all over top of pasta and stir well.
Place meatballs over top of pasta in baking dish, nestling them into pasta and sauce. Sprinkle shredded mozzarella cheese all over top of dish.
Add meatball casserole to oven, and bake for 35 to 40 minutes, until meatballs are fully cooked thru. Serve warm.