In a pot, place the chicken breasts and cover them with water. Bring to a boil, then simmer until the chicken is no longer pink in the center and the juices run clear, about 15 minutes.
In a separate large pot, boil lightly salted water. Add the penne pasta and cook until al dente, about 11 minutes. Drain the pasta.
Drain the cooked chicken and cut it into bite-size pieces.
In a large bowl, combine the chicken, cooked penne, Alfredo sauce, 1/2 cup of mozzarella cheese, milk, onion powder, and garlic powder. Mix well.
Transfer the mixture into a 1.5-quart casserole dish. Top with the remaining mozzarella and Cheddar cheese.
Bake in the preheated oven until the cheese is melted and bubbly, about 25 minutes.
Notes
You can add cooked broccoli or spinach to the casserole for extra flavor and nutrition.