01 - Preheat oven to 400°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - In a large bowl, combine ground turkey, breadcrumbs, egg, grated Parmesan cheese, minced garlic, chopped parsley, dried oregano, salt, and black pepper. 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 the meatballs. Sprinkle with dried basil and red pepper flakes if using. Bake uncovered for 20 minutes.
05 - While the meatballs bake, toss bread cubes with melted butter, minced garlic, and chopped parsley in a large bowl until thoroughly coated.
06 - Remove the baking dish from the oven. Sprinkle shredded mozzarella and grated Parmesan cheese over the meatballs and sauce. Arrange garlic bread cubes in an even layer on top.
07 - Return to oven and bake for 15 minutes, or until bread cubes are golden brown and cheese is melted and bubbly.
08 - Let the casserole rest for 5 minutes before serving to allow flavors to meld and make serving easier.