Sprinkle tomato slices evenly with salt and pepper; set aside.
Brush both sides of bread slices with olive oil.
Grill bread, without grill lid, over medium heat (300° to 350°) 2 to 3 minutes on each side or until lightly browned.
Grill onion, covered with grill lid, over high heat (400° to 500°) 8 to 10 minutes on each side or until tender and browned.
Stir together mayonnaise and pesto; spread evenly on 1 side of each bread slice. Top evenly with tomato and onion slices; sprinkle with olives. Sprinkle with mint, if desired.
* 4 (1-inch thick) French bread slices may be substituted.