Coat A Piece Of Heavy-Duty Foil (About 14 In. X 14 In.) With Nonstick Cooking Spray. Place Fillets On Foil; Brush With Lemon Juice. Crimp Foil, Forming Edges. Place Foil Flat On The Grill (Do Not Seal). Grill, Covered, Over Medium-Hot Heat For 4 Minutes. Combine Parmesan Cheese, Butter, Mayonnaise, Onions And Salt; Brush Over Fillets. Grill 3-4 Minutes Longer Or Until Fish Flakes Easily With A Fork.