Cut the fish into 2 inch lengths and remove the bones. Put the fish, the bones and the finely chopped onions in a saucepan with the cold water, seasoning with the salt and paprika. Bring to the boil, and simmer till the fish is tender. Remove the pieces of fish carefully from the stock, put them in a basin or a dish and strain the stock over them. Put in a cold place till the stock has set in a jelly.