Snow Covered Broccoli Trees

Instructions:
Combine ½ C. mayonnaise or salad dressing, 2 Tbs. margarine (melted), 1 Tbs. grated onion, ¼ Tsp. each: salt and dry mustard, 1 Tbs. prepared horseradish. Chill for 3-4 hours. Serve over steamed broccoli.