Preheat oven to 350°F. Mist a 9-by-13-inch baking dish with cooking spray. Place cauliflower in baking dish and toss with bell pepper. Season with salt and pepper, if desired.
In a blender, blend cream cheese, chicken broth, onion, garlic and 1/4 cup Parmesan until smooth and creamy. Pour over cauliflower mixture. In a large bowl, stir together Cheddar, bread crumbs, parsley and 1/4 cup Parmesan. Spread evenly atop cauliflower mixture. Drizzle with butter.
Bake until gratin is brown and bubbling and cauliflower is tender, 45 to 60 minutes. Let stand for 10 minutes before serving.