Remove part of cauliflower stem, leaving whole head intact. Bring 1 inch of water to a boil in large saucepan or Dutch oven. Place cauliflower, stem end down, in saucepan; cover and steam 15-20 minutes or until crisp-tender.
Meanwhile, in small bowl combine mayo, mustard and onion; mix well. Spoon and spread over hot cauliflower. Cover; let stand a few minutes. Sprinkle with sunflower seeds.
Nutrition Info Per Serving: Calories 61.5 Total Fat 3.3 g Saturated Fat 0.1 g Polyunsaturated Fat 2.1 g Monounsaturated Fat 0.5 g Cholesterol 0.0 mg Sodium 122.1 mg Potassium 313.7 mg Total Carbohydrate 7.0 g Dietary Fiber 2.7 g Sugars 2.3 g Protein 2.4 g