Put the fish head and bones, white wine, lemon juice, onions, bay leaf, parsley and thyme into a large heavy saucepan. Bring to the boil, stirring frequently until the mixture is reduced by half.
Pour in the water and bring to the boil again. Carefully skim the top. Simmer for 20 minutes then strain into another large pan. Let the stock cool then skim again.
Can be used as a base for fish soup or in recipes calling for fish stock.