Wash fish bones in cold water for a few minutes no head.
Melt the butter in a thick bottomed pan
Add the sliced onions, the well-washed fish bones and remainder of the ingredients except the water.
Cover with greaseproof paper (cartouche) and a lid and sweat (cook gently without colouring) for 5 minutes
Add the water, bring to the boil, skim and simmer for 20 minutes,
Longer cooking time will spoil the flavour
