1. Preheat the oven to 375°F.
2. Cook the penne pasta al dente according to package instructions, then drain.
3. Steam or blanch the broccoli florets until slightly tender, then drain.
4. Shred or chop the rotisserie chicken into bite-sized pieces.
5. In a saucepan, melt butter over medium heat and sauté minced garlic.
6. Stir in flour and cook for about a minute to form a roux.
7. Gradually whisk in milk or cream, stirring continuously until the sauce thickens.
8. Add Parmesan cheese and stir until melted and smooth.
9. Season the sauce with garlic powder, Italian seasoning, salt, and pepper to taste.
10. In a large mixing bowl, combine the cooked pasta, chicken, broccoli, and Alfredo sauce.
11. Transfer the mixture into a greased casserole dish, spreading it evenly.
12. Sprinkle shredded mozzarella cheese on top for a gooey, golden crust.
13. Bake in the preheated oven for 15-25 minutes or until hot and bubbly.
14. Optionally, broil for 2-3 minutes at the end for a golden-brown top.
15. Let the casserole cool slightly before serving.
16. Serve warm and enjoy your creamy Broccoli Chicken Alfredo Casserole.
17. Store leftovers in an airtight container in the refrigerator or freeze for later.