Preheat oven to 350 and spray a 13 x 9 baking dish with non-stick spray. In a food processor, combine cauliflower, egg whites and nutritional yeast and process until cauliflower is “riced.”  Remove from the food precessor, and place cauliflower mixture in a large bowl. Add your cream cheese, chicken, onion, garlic, salt and pepper, and mix until well combined. Place mixture in your prepared pan and top with the mozzarella cheese. Bake for 45-50 minutes or until casserole is set and cheese is golden brown. Enjoy!