Break cauliflower into small pieces. In a good-sized saucepan, steam cauliflower, garlic and leeks in water until completely tender, about 20 to 30 minutes. While cauliflower is hot, puree until the vegetables resemble mashed potatoes. (Use a food processor, or if you prefer a smoother texture, use a blender. Process only a small portion at a time, holding the blender lid on firmly with a hand towel.) Add a little hot water if vegetables seem dry. Stir in margarine and pepper to taste.

From the Mayo Clinic 

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