Sweet and Sour Mackerel

This should marinate for a minimum of 8 hours but can be up to 48 which does intensify the flavour. I did just for 8 hours today but normally do for 24 - lovely with fresh bread.

4 medium smoked mackerel fillets (peppered is ok)
1 large Spanish onion
2 tbsp dark brown sugar
7 fl oz white vinegar
14 fl oz water
2 bay leaves

Cut fillets into 3-4 diagonal pieces and lay in shallow dish
Top with thinly sliced onion
Sprinkle with brown sugar and tuck in the bay leaves
Mix vinegar and water and pour over fish
Cover with cling film and leave to marinate in cold place