Rinse the fish bones and heads thoroughly, then place them in a large saucepan or stockpot. Add the onion, parsley, bay leaves, carrots, celery, peppercorns and water. Bring to the boil, then reduce the heat and simmer gently for about 30 minutes, skimming the froth from the surface as it appears.
Remove from the heat and leave to cool, then strain the stock through a fine sieve into a heatproof bowl. Discard the fish trimmings and vegetables.
Use the stock at once or set aside to cool. When cold, refrigerate the stock and use within 3-4 days. Alternatively, the stock can be frozen in measured quantities for later use.
Fresh stocks will keep in the refrigerator for 3–4 days or in the freezer for up to 6 months.
• For special-occasion fish dishes, use 300 ml white wine and just 1 litre of water. • Make a SHELLFISH STOCK using prawn, crab, lobster or mussel shells, instead of the white fish trimmings.