Melt half the butter in a frying pan for the stuffing. Add the breadcrumbs.
Fry and stir the mixture over medium-high heat until the breadcrumbs are browned. Remove to a mixing bowl.
Melt remaining butter in the frying pan and cook the onions, celery and garlic until soft and onions are transparent
Add to the breadcrumbs in the mixing bowl, then stir in prawns, crab, parsley, salt and pepper; Toss together gently.
Cut aluminium foil to form a double-thickness large rectangle. Lay the fish fillets on the foil. Mound the stuffing on top of the fillets.
Curl up the edges of the foil to form a tray. In the barbecue, arrange the preheated charcoals to either side. Test for medium heat above the centre of the grill. Place the fish in foil in the centre of the cooking grill. Cover and cook for 20 to 25 minutes or until the fish flakes easily.