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 13 g Cholesterol 70 mg Sodium 348 mg Potassium 374 mg Total Carbohydrate 2 g Dietary Fiber 0 g Sugars 0 g Protein 26 g