Rinse the fish bones and heads thoroughly, then place them in a large pot or stockpot. Add the onion, parsley, bay leaves, carrots, celery, peppercorns and water. Bring to a 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 cool and chill or freeze.
Tip: Fresh stocks will keep in the refrigerator for 3-4 days or in the freezer for up to 6 months.
Variations: For special-occasion fish dishes, use 11⁄4 cups white wine and just 4 cups of water. Make a shellfish stock using shrimp, crab, lobster or mussel shells, instead of the white fish trimmings.