01 - Preheat oven to 400°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Combine ground turkey, breadcrumbs, egg, Parmesan, minced garlic, parsley, oregano, salt, and pepper in a large bowl. Mix gently until just combined, being careful not to overwork the meat.
03 - Shape the mixture into 1 ½-inch meatballs, approximately 18-20 total. Arrange them evenly in the prepared baking dish.
04 - Pour marinara sauce evenly over all the meatballs. Sprinkle with dried basil and red pepper flakes if using.
05 - Bake uncovered for 20 minutes to partially cook the meatballs and heat through the sauce.
06 - While meatballs bake, toss bread cubes with melted butter, minced garlic, and fresh parsley in a large bowl until evenly coated.
07 - Remove the dish from the oven. Sprinkle mozzarella and Parmesan cheese over the meatballs. Arrange garlic bread cubes in an even layer on top.
08 - Return to oven and bake for 15 minutes, or until bread cubes are golden brown and cheese is fully melted and bubbly.
09 - Let the casserole rest for 5 minutes before serving to allow the flavors to settle and make serving easier.