1. Preheat oven to 375°. Cook cauliflower according to pkg directions and drain. 2. Blend sour cream, cheese, bouillon and mustard. Stir in cauliflower and transfer to 13 x 9 pan. 3. heat butter over medium heat. Add stuffing mix and walnuts (optional), stir until lightly toasted. Sprinkle over casserole. 4. Bake uncovered for 17-20 minutes or until heated through.