Broil fillets on top oven rack (about 6" from heat) with door partially cracked/opened, about 5 minutes or until fish is cooked through and flakes easily.
Mix mayo, cheese, onion and butter together in a small bowl.
Spread mayo mixture evenly over cooked fillets and return to the oven broiling an additional 1-2 minutes, until browned and bubbling.
Nutrition Info Per Serving: Calories 226 Total Fat 12.86 g Saturated Fat 4.0 g Polyunsaturated Fat 3.9 g Monounsaturated Fat 2.8 g Cholesterol 70.0 mg Sodium 348.0 mg Potassium 374.0 mg Total Carbohydrate 1.8 g Dietary Fiber 0.3 g Sugars 0.3 g Protein 25.8 g