Preheat the oven: Set to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Cook the veggies: Steam or microwave broccoli and cauliflower until crisp-tender, about 3–4 minutes.
Drain well and pat dry. Excess water will thin the sauce.
Prep the chicken: Use rotisserie or leftover cooked chicken. Shred or cube into bite-size pieces.
Season lightly with salt and pepper.
Start the Alfredo base: In a large skillet over medium heat, melt butter. Add garlic and cook 30–60 seconds until fragrant, not browned.
Add liquids: Pour in heavy cream and chicken broth. Whisk in cream cheese until smooth.
Simmer gently 3–4 minutes to thicken.
Stir in cheeses and seasoning: Add Parmesan and 1 cup mozzarella. Whisk until melted. Season with Italian seasoning, onion powder, red pepper flakes if using, plus salt and black pepper.
Taste and adjust.
Combine: In a large bowl, toss chicken, broccoli, and cauliflower with the sauce until everything is evenly coated.
Assemble: Transfer mixture to the baking dish. Top with remaining 1/2 cup mozzarella (or more for extra coverage).
Bake: Bake 18–22 minutes until bubbly around the edges. For a golden top, broil 1–2 minutes at the end.
Watch closely.
Rest and serve: Let stand 5–10 minutes so the sauce sets. Garnish with parsley. Serve warm.