01 - Preheat oven to 400°F. Grease a 9x13-inch baking dish lightly with cooking spray or butter.
02 - Combine ground turkey, breadcrumbs, egg, Parmesan cheese, minced garlic, parsley, oregano, salt, and pepper in a large bowl. Mix gently until just combined. Form mixture into 1 ½-inch meatballs, approximately 18-20 total, and arrange in the prepared baking dish.
03 - Pour marinara sauce evenly over the meatballs. Sprinkle with dried basil and red pepper flakes if desired. Bake uncovered for 20 minutes.
04 - While meatballs bake, toss bread cubes with melted butter, minced garlic, and parsley in a large bowl until evenly coated.
05 - Remove baking dish from oven. Sprinkle mozzarella and Parmesan cheese evenly over meatballs and sauce. Arrange garlic bread cubes in a single layer on top.
06 - Return to oven and bake for 15 minutes until bread cubes are golden brown and cheese is fully melted and bubbly.
07 - Let casserole rest for 5 minutes before serving to allow flavors to settle and make serving easier.