01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with cooking spray or oil.
02 - Spread the cooked chicken evenly across the bottom of the prepared baking dish.
03 - Pour marinara sauce over the chicken in an even layer. Sprinkle dried Italian herbs evenly across the sauce if using.
04 - Distribute shredded mozzarella cheese and half of the grated Parmesan cheese over the sauce layer.
05 - In a small bowl, combine panko breadcrumbs with melted butter. Mix until crumbs are evenly moistened. Stir in chopped parsley if using.
06 - Sprinkle the breadcrumb mixture evenly over the cheese layer. Top with remaining Parmesan cheese.
07 - Bake uncovered for 25 to 30 minutes, until the topping is golden brown and the cheese is bubbly.
08 - Let the casserole stand for 5 to 10 minutes before serving to allow the layers to set.