Preheat your oven to 375F and move the rack to the top third of the oven.
Boil a large, salted pot of water for the pasta, and cook it for 2 minutes less than the package directions indicate. Add the broccoli to the pot 2 minutes before the pasta is done (I cook the penne for 8 min and add the broccoli in at the 6 min mark). Drain and transfer to a 9x13 casserole dish.
To a medium bowl, add the cream of chicken soup, milk, sour cream, parmesan, Italian dressing mix, onion powder, and garlic, and stir until smooth.
Add the chicken and frozen mixed vegetables to the casserole dish, and then pour the sauce mixture over top. Toss until everything is well coated.
Smooth it out in an even layer, and then top it with the mozzarella.
Bake, uncovered, for 20-25 minutes until hot and bubbly. Optional: broil it for a few minutes to brown the cheese (watch it closely so it doesn't burn). Season with salt & pepper as needed.