Heat the oven to 375°F and coat a 9-by-13-inch baking dish with cooking spray.
Add the dry penne, marinara, broth, Italian seasoning, salt and black pepper to the dish and stir until every noodle is wet.
Cover the dish tightly with foil and bake for 45 minutes.
Uncover and stir from the edges toward the center; test a noodle, and if it still has a hard core, add 1/4 cup hot water, reseal and bake for 5 to 10 minutes more.
Scatter the mozzarella over tender pasta and bake uncovered for 10 minutes, until the sauce bubbles and the cheese has browned spots.
Rest the casserole for 8 minutes so the sauce clings before serving.