In a hot pan, add butter, then sauté together carrots, celery stick, and leeks till soft. Then add the fish, wine and bouquet garni, bring to a boil. Skim for any sediment. Then add the peppercorns and lemon slices, boiling all the while. Simmer the stock for about 30 minutes then strain through a soft muslin cloth and remove the bouquet garni. Cool and use as the base for this sauce.