Lightly coat the mango slices and onion discs in olive oil and sprinkle with salt and pepper. Place mango slices on a medium high grill approximately 2 minutes on each side. Grill onions approximately 2 to 4 minutes on each side, being careful to turn only once (this will help keep them together and avoid pieces falling through the grill). When mangos have nice grill marks after a couple minutes on each side, remove from grill. Once onions have slightly charred, but still retain moderate firmness, remove from grill.
Chop the mango and onion and place in large mixing bowl. Combine all of the above ingredients including juice of lime and orange and add salt and pepper to taste. In the mixing process a large amount of juice will gather at the bottom of the bowl. I recommend draining the juice, and placing the salsa in a refrigerator, covered for approximately 20 minutes. Then, use the salsa on top of grilled fish or serve as a side dish.