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 to avoid tough meatballs.
03 - Shape mixture into 1 ½-inch meatballs, yielding approximately 18-20 pieces. Arrange evenly in the prepared baking dish.
04 - Pour marinara sauce over the meatballs. Sprinkle with dried basil and red pepper flakes if using.
05 - Bake uncovered for 20 minutes at 400°F.
06 - While meatballs bake, toss bread cubes with melted butter, minced garlic, and parsley in a large bowl until evenly coated.
07 - Remove dish from oven. Sprinkle mozzarella and Parmesan cheese over meatballs. Arrange garlic bread cubes in an even layer on top.
08 - Return to oven and bake for 15 minutes, until bread cubes are golden brown and cheese is fully melted and bubbly.
09 - Let casserole rest for 5 minutes before serving to allow flavors to meld.