Preheat the oven to 375°F (190°C).
Cook the whole-grain penne according to package directions until al dente, then drain and set aside.
In a 9 x 13-inch casserole dish, combine the cooked pasta, marinara sauce, diced tomatoes (if using), and the bite-size chicken pieces; stir until evenly mixed.
Cover the casserole and bake for 30 minutes.
While the casserole bakes, mix the Panko, grated Parmesan, and Italian seasoning in a small bowl to make the topping.
After 30 minutes, remove the casserole from the oven, stir in the shredded mozzarella, and evenly sprinkle the breadcrumb mixture over the top.
Return the casserole to the oven uncovered and bake an additional 10–15 minutes, or until the chicken is cooked through and the topping is golden and crispy.
Let rest a few minutes before serving.
