Bring a large pot of salted water to a boil. Add the whole grain penne pasta and cook according to package instructions until al dente. Drain and set aside.
Heat a non-stick skillet over medium heat. Add the bite-sized chicken pieces and cook until no longer pink in the center, about 5-7 minutes. Season with salt and pepper to taste.
In a large mixing bowl, combine the cooked pasta, marinara sauce, diced tomatoes (if using), and cooked chicken. Stir until everything is well-coated and evenly distributed.
Transfer the pasta mixture into a greased 9x13-inch baking dish and spread it out evenly.
Sprinkle the shredded skim mozzarella cheese evenly over the pasta mixture. In a small bowl, combine the Panko bread crumbs, grated Parmesan cheese, and Italian seasoning. Sprinkle this mixture over the cheese layer.
Preheat oven to 375°F (190°C). Bake the casserole for 25-30 minutes, or until the cheese is bubbly and the top is golden brown.
Remove from oven and let sit for a few minutes before serving. Enjoy hot, optionally garnished with fresh herbs.