Place the lemon, lime and onion slices in a shallow dish large enough to hold the fish in one layer. Add the parsley, orange juice, oil, garlic, mustard, rosemary and pepper then stir until well mixed. Add the fish and turn it to cover with the mixture. Cover the dish and leave to marinate in the fridge for about an hour.
Preheat the grill to high. Cover the rack in the grill pan with foil and perforate it. Coat the foil with vegetable oil spray. (Do not use the spray near the hot grill.)
Using a fish slice, transfer the fish to the foil-covered grill rack. Strain the marinade into a small saucepan; reserve the lemon, lime and onion slices. Warm the marinade over low heat.
Grill the fish, about 13cm below the heat, for 6–8 minutes, brushing frequently with the marinade. Carefully turn the fish over and brush with the marinade. Arrange the lemon, lime and onion slices in an even layer on top of the fish, then return to the grill and cook for a further 6–8 minutes. Serve hot.