Cut two slanting slits on both sides of the fish. Rub lightly with salt and lime juice. Combine marinda ingredients in a bowl. Brush fish with marinade and set aside for one to two hours in the refrigerator. Turn over several times to get the fish evenly coated.
Place fish on a piece of lightly-oiled aluminum foil. Brush with remaining marinade and melted margarine or ghee. Grill or bake for 30 to 40 minutes at 170ºC or until cooked (the fish is cooked if its eyes pop out).
Serve fish with an extra sprinkling of garam masala and lime juice and garnish with raw onion rings and tomato slices.