PLACE TUNA in shallow non-metal dish.
1.Cut onions in half; slice finely.
Sprinkle over fish.
Combine wine, sugar, salt and pepper in jug; mix well.
2.Pour marinade over fish.
Cover and refrigerate several hours.
Prepare and heat the barbecue 1 hour before cooking.
Drain fish and onion, reserving marinade.
3.Cook tuna steaks and onions on hot lightly greased barbecue flatplate 8-10 minutes or until lightly browned and just cooked through.
Pour marinade over tuna and onions a little at a time during cooking.