1. Coat the grill rack with cooking spray; heat the grill to medium.
2. Pour oil in a small bowl. Dip onion wedge and plum halves in oil to just coat the surface.
3. Place plums, cut side down, onion and bread on grill; cook about 4 minutes, or until bread is lightly toasted. Remove bread from grill; turn plums and onion and cook 4 minutes more, or until lightly grilled.
4. Chop plums and onion and toss with salt and cilantro. Spoon over grilled bread.