Peel and finely chop the onions, shallots and carrots. Roughly crush the fish trimmings, then put them in a large pan with the onions, shallots and carrots. Add the bouquet garni, and the half lemon, cut in half again.
Pour in a large glass of white wine and 2 pints (1.2ltr) of water. Add a generous pinch of sea salt and a few twists of freshly ground black pepper. Bring to boil, then turn the heat down and simmer without covering, allowing the stock to reduce for one hour.
When the liquid has cooled, strain through a sieve (strainer), retaining the liquid and disposing of the solids. Store in the fridge or freezer.