After you have removed the sausage from the pan, add half of the butter to the pan. Fry the onion and garlic for 2 minutes on low heat. Add the leek and celery, turn up the heat and stir-fry for 2 minutes. Deglaze with the bock, add the sausage (including moisture) and braise, covered, for 6 – 8 minutes on low heat. If necessary, remove the lid of the pan in the last 2 minutes to allow excess moisture to evaporate.