Preheat your oven to 350 degrees F. Grease a 9x13 baking dish or spray with cooking spray.
Stir cooked pasta, cooked chicken, 2 cups marinara sauce, diced tomatoes with juices, 1 cup mozzarella cheese, ½ cup Parmesan cheese, Italian seasoning and pepper in a large bowl until combined.
Spoon into prepared baking dish. Spoon remaining 1 cup of marinara sauce over mixture and spread as evenly as possible with back of spoon. Cover tightly with aluminum foil.
Bake for 30 minutes, remove from oven and carefully remove foil. Sprinkle evenly with remaining 1 cup mozzarella cheese, remaining ½ cup Parmesan cheese, and panko breadcrumbs.
Return to oven and bake (uncovered) for an additional 10 minutes, or until cheese is melted.
Garnish with chopped fresh parsley and/or basil, if desired.