Heat the oven to 400°F and coat a 9-by-13-inch baking dish with cooking spray.
Combine the rigatoni, chicken, pesto, broth, milk, salt and black pepper in the dish.
Stir thoroughly, press the pasta and chicken beneath the liquid and seal tightly with foil.
Bake for 42 minutes, then stir and check that the chicken reaches 165°F and the center pasta is tender.
Add 1/4 cup hot water, reseal and bake for 5 to 10 minutes if the rigatoni still has a hard center.
Cover with mozzarella, bake uncovered for 6 minutes and rest for 8 minutes before serving.