Always remove the eyes of the fish with a teaspoon and discard before commencing with the stock.
Place 2kg of fish bones and trimmings in a pot and cover with salted water. Set aside for 5 minutes and discard the water.
Place the fish bones and trimmings in a large pot and add 1 chopped onion, 1 chopped leek, 1 chopped celery stick, a few peppercorns, 1 bay leaf and the juice of 2 lemons.
Pour in 2L of water and bring to a boil, skimming any scum from the surface. Reduce heat and simmer, about 30 minutes.
Strain through a sieve, freeze when cold and use as required.