Rub olive oil over tuna, bell pepper and onion. Season all with salt and pepper. Place onions and peppers on grilled until charred. Place in a paper or plastic bag to soften for 5 minutes. Meanwhile, place tuna steaks on the grill and grill both sides, but not past medium-rare. Just before done, melt cheese over tuna. Combine mayonnaise with mustard, garlic and lime juice. Spread mayonnaise mixture on both sides of each bun. Place lettuce on bottom of bun. Place tuna on lettuce and top with peppers and onions.