01 - Preheat oven to 400°F and 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, then form into 1 ½-inch meatballs, yielding approximately 18-20 pieces.
03 - Place meatballs in the prepared baking dish and pour marinara sauce evenly over them. Sprinkle with dried basil and red pepper flakes if desired.
04 - Bake uncovered for 20 minutes to partially cook the meatballs and heat the sauce.
05 - While meatballs bake, toss bread cubes with melted butter, minced garlic, and chopped parsley in a large bowl until evenly coated.
06 - Remove dish from oven. Sprinkle mozzarella and Parmesan cheese over meatballs, then arrange garlic bread cubes in an even layer on top.
07 - Return to oven and bake for 15 minutes until bread cubes are golden brown and cheese is fully melted and bubbly.
08 - Let the casserole rest for 5 minutes before serving to allow flavors to meld and make serving easier.