Break cauliflower into sections. Cook for 10 minutes in boiling, salted water. Drain well. Combine cauliflower with 2 tablespoons butter and onion, cheese, sour cream and salt. Spoon into a 1 1/2 quart casserole.
Melt remaining butter and add garlic and horseradish. Pour over the cauliflower mixture. Bake, uncovered at 350 degrees for 30 minutes or until heated through.