Break the cauliflower into florets and steam until tender, about 10 minutes. While cauliflower is steaming, sauté the onion in the butter. When the cauliflower and the onions are done, place everything in a large mixing bowl. Mash all the ingredients together and place them in a buttered 9? x 12? casserole or a large baking dish. Bake at 350° F for 30-45 minutes, or until cheese is completely melted and the casserole is heated through. Serve hot.