Pre-heat the
oven to 160C/325F/Gas 3. Clean and bone the fish.
Cut each fish into two fillets.
Roll
up the fillets tightly and place them closely packed
together in an ovenproof dish so that they can't
unroll.
Pour the vinegar over the fish and add just enough
water to cover them.
Add the spices and onion, cover and cook for 1 hour.
Leave to cool with the liquid. To make the dressing,
combine all the ingredients and shake well; serve
with the fish.