Preheat a barbecue for medium heat and place the rack 8cm from the coals.
In a medium bowl mix together the jalapenos, spring onions, coriander, capsicum, mango, olive oil, lime juice, garlic salt and black pepper; set aside.
Lightly coat four squares of foil with olive oil or cooking spray. Place fish diagonally on the foil and stuff each with 1/4 of the mango stuffing. If it doesn't all fit inside the fish then just place the remainder on top of the fish. Fold the corners of the foil over the head and tail of the fish then fold the remaining corners over the body of the fish.
Cook the packets on both sides for about 20 minutes total, until the fish has cooked and flakes easily.