In A Small Skillet Over Medium Heat, Cook And Stir The Onion, Lime Juice, Brown Sugar, Butter And Mustard For 4-5 Minutes Or Until Onion Is Tender And Liquid Is Almost Evaporated. Place Each Fillet On A Double Thickness Of Heavy-Duty Foil (About 12 In. Square). Top With Onion Mixture. Fold Foil Over Fish And Seal Tightly. Grill, Covered, Over Medium Heat For 6-8 Minutes Or Until Fish Flakes Easily With A Fork. Meanwhile, Brush Oil Over Both Sides Of Bread. Grill For 30-60 Seconds On Each Side Or Until Lightly Browned. Top Each Slice Of Bread With A Slice Of Cheese; Grill 1 Minute Longer Or Until Cheese Is Melted. Top With Salsa. Open Foil Packets Carefully To Allow Steam To Escape. Serve Fish With Bruschetta.