Preheat the oven: Heat the oven to 375°F. Lightly grease a 9 × 13-inch baking dish.
Season the chicken: Pat the chicken dry and cut it into evenly sized pieces. Toss with olive oil, garlic powder, Italian seasoning, black pepper, and salt.
Prepare the broccoli: Cut fresh broccoli into bite-sized florets. If using frozen broccoli, thaw and drain it thoroughly to prevent excess moisture.
Arrange the ingredients: Spread the seasoned chicken and broccoli in the prepared baking dish, keeping the ingredients in a reasonably even layer.
Add the Alfredo sauce: Pour the Alfredo sauce over the chicken and broccoli. Gently toss or turn the ingredients so the sauce coats them evenly.
Add the cheese: Sprinkle Parmesan over the mixture, followed by shredded mozzarella.
Bake the casserole: Bake for approximately 30–40 minutes, checking earlier if the chicken pieces are small. If the cheese browns too quickly, cover loosely with foil.
Check doneness: Use a food thermometer to confirm that the thickest chicken pieces reach an internal temperature of 165°F.
Rest and serve: Let the casserole rest for about 5 minutes before serving. Spoon the creamy chicken, broccoli, and melted cheese onto plates.