Peel and finely chop the onion. Rinse the lentils. Heat the oil in a large pot. Fry the onion until transparent. Stir in tomato paste, sweat briefly. Add about 1 1/2 l water, stock, bay leaf and lentils, bring to the boil. Cover and cook for about 25 minutes.
In the meantime, clean the celery, pick up some greenery for garnishing, wash and chop into small pieces. Peel, wash and chop the potatoes. Add both to the lentils, cook everything else for about 20 minutes
Cut the sausages into diagonal pieces. Add to the stew about 5 minutes before the end of the cooking time. Season to taste with vinegar, salt, pepper and sugar. Garnish with the remaining celery green