1. Cut the fish bones and trimming into equal-sized pieces. Place them in a bowl of cold, salted water to absorb moisture and remove any blood. Drain and place the bones and trimming into a large stockpot.
2. Cut carrots, celery and onion into pieces ofequal size and add them( and any other flavourings you wish) into the pan with the fish. Cover with water turn up the heat and bring to the boil.
3. Once the stock reaches the boil, lower the leat and simmer for 2o minutes ( any longer and the stock wikk start to become bitter). Skim off the scum that rises to ther surface with a slotted spoon.
4. Ladle through a fine sieve, pressing the solids against the sieve to extract any extra liquid. Season to taste with salt and freshly ground black pepper, let cool and refrigetate for up to 3 days.