Preheat and prep: Heat the oven to 400°F (200°C).
Lightly grease a 9x13-inch baking dish with cooking spray or a little butter.
Build the base: Add the uncooked pasta, chicken, minced garlic, Italian seasoning, onion powder, pepper, red pepper flakes, and salt to the dish. Give it a quick toss so the spices are spread evenly.
Add the liquids: Pour in the chicken broth, heavy cream, and the jar of Alfredo sauce. Stir well to coat all the pasta and distribute the sauce.
Cheese it up: Stir in half of the Parmesan and half of the mozzarella.
Dot the top with the butter pieces for extra richness.
Cover and bake: Tightly cover the dish with foil. Bake for 30 minutes to let the pasta soften and the sauce begin to thicken.
Uncover and finish: Remove the foil, stir the pasta to prevent sticking, and fold in peas or spinach if using. Sprinkle the remaining mozzarella and Parmesan over the top.
Bake uncovered for another 10–15 minutes, until the pasta is tender and the top is bubbly and golden around the edges.
Rest and serve: Let the bake rest for 5–10 minutes so the sauce sets slightly. Garnish with chopped parsley or basil. Taste and adjust seasoning if needed before serving.