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, Parmesan cheese, minced garlic, parsley, oregano, salt, and pepper. Mix gently until just combined. Form into 1 ½-inch meatballs, yielding about 18-20 pieces, and arrange them in the prepared baking dish.
03 - Pour marinara sauce evenly over the meatballs. Sprinkle with dried basil and red pepper flakes if using.
04 - Bake uncovered for 20 minutes to partially cook the meatballs.
05 - While the meatballs bake, toss bread cubes with melted butter, minced garlic, and parsley in a large bowl until evenly coated.
06 - Remove the baking dish from the oven. Sprinkle mozzarella and 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 the flavors to meld.