Lightly sweat off the vegetables in a little olive oil - do not colour. Add the fish bones, peppercorns, lemon, bay leaves and wine, top up with water, bring to a gentle boil and skim off the frothy impurities. Simmer for 20 minutes, reducing by a third. Then remove from the heat and allow to stand for 10 to 20 minutes to allow the sediment to settle. Pass through a fine sieve or chinois and use as required.