Preheat your oven to 375°F (190°C). Lightly grease a 9x9-inch or similar 2-quart baking dish.
Cook the pasta in a large pot of salted water according to package directions for al dente. Drain thoroughly.
While the pasta is cooking, make the topping. In a small bowl, stir together the Panko breadcrumbs, Parmesan cheese, melted butter, parsley, and garlic powder until combined.
In a large bowl, mix the drained pasta with 2 cups (475 ml) of the marinara sauce, the shredded chicken, drained diced tomatoes, and Italian seasoning. Stir well to coat everything evenly.
Transfer the pasta mixture to the prepared baking dish and spread it out in an even layer.
Pour the remaining 1 cup (235 ml) of marinara sauce over the top of the pasta.
Sprinkle the shredded mozzarella cheese evenly over the sauce, followed by the breadcrumb topping.
Bake for 23 to 25 minutes, until the casserole is hot and bubbly and the topping is golden brown. A thermometer inserted into the center should read at least 165°F (74°C).
Let the casserole rest for 5 minutes before serving.