Prep the oven and pan. Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with olive oil or butter.
Par-cook the veggies. Toss the cauliflower (or broccoli) with 2 tablespoons olive oil or butter, salt, and pepper. Spread on a sheet pan and roast for 12–15 minutes until just tender.
This prevents a watery bake.
Make the Alfredo sauce. In a large skillet over medium heat, melt 3 tablespoons butter. Add garlic and cook for 30–60 seconds until fragrant. Pour in the heavy cream and chicken broth.
Simmer 3–4 minutes, stirring, until slightly thickened.
Add the cheeses and seasoning. Reduce heat to low. Stir in Parmesan, Italian seasoning, red pepper flakes (if using), and a pinch of salt and pepper. Cook 1–2 minutes until smooth and creamy.
Taste and adjust seasoning.
Combine the bake. In a large bowl, mix the roasted veggies, cooked chicken, and any optional add-ins. Pour the Alfredo sauce over the top and toss to coat evenly.
Assemble in the dish. Transfer everything to the prepared baking dish. Sprinkle mozzarella and a little extra Parmesan over the top for that golden, cheesy crust.
Bake. Bake uncovered for 18–22 minutes, until bubbly around the edges and lightly browned on top.
If you want extra color, broil for 1–2 minutes at the end. Keep an eye on it.
Rest and serve. Let it rest for 5 minutes so the sauce sets slightly. Garnish with chopped parsley and serve warm.