Ignite coals about 35 minutes before you intend to begin cooking (you will need about 20 to 30 long burning briquets). When coals are covered with gray ash, arrange in a single layer to underlie area being used on grill. Knock ash off coals. Adjust grill to 4 to 6 inches above low-glowing coals.
Cut heavy-duty foil the same size as fish fillet. Place fillet, skin side down, on foil.
Combine butter, wine, lemon juice, and soy sauce.
Place fish on grill, foil side down, and brush generously with butter sauce.
Cover barbecue with its hood (or fashion a hood out of heavy-duty foil and place on grill to enclose fish).
Cook, basting with butter sauce about every 5 minutes, until fish flakes readily when prodded in thickest portion with a fork (allow about 10 minutes cooking time for each inch of thickness).
To serve, slide fish, still on foil, onto a serving plate or board.
Garnish with parsley and lemon.
Slice fish to the skin and lift each serving away from skin.
Reheat any remaining butter sauce and pass at table to serve over fish.