In a large stock pot, lightly soften the vegetables in the butter, without letting them colour. Add the garlic, bay leaf, thyme, peppercorns and chopped carcasses. Cover with the cold water, season with salt and bring to the simmer, skimming off the impurities all the time. Allow the stock to simmer for 2–3 hours.
Drain through a sieve. The stock is now ready to use and will keep well,