Rinse the fish under plenty of cold running water and pat dry on kitchen paper.
Cut into 1/2 inch (1.25 cm) slices.
Discard the head and tail.
Sprinkle salt and pepper into the base of a dish.
Add the slices of fish.
Pour the vinegar over and season with salt and pepper.
Leave to marinate for at least 1 hour.
Put the onions into a dish large enough to hold the fish.
Cook, uncovered, for 3 minutes on HIGH.
Add the curry powder, concentrated tomato juice, sour cream (creme fraiche) and 1-2 tablespoons of the fish marinade to the onions and stir well.
Drain the slices of fish and arrange on top of the onion mixture.
Cover and microwave on HIGH for 5 minutes.