Peel, wash and chop the potatoes, carrots and kohlrabi. Wash and clean the cauliflower and cut into florets. Peel and finely chop the onion. Heat 1 tablespoon of oil in a saucepan.
Sauté onion cubes, potatoes, carrots, kohlrabi and cauliflower in it. Stir in vegetable stock. Add 1 litre of water, bring to the boil and cook for about 20 minutes. Clean, wash and cut the leek into rings and add to the vegetables 10 minutes before the end of the cooking time.
Heat 1 tablespoon of oil in a frying pan. Press the sausage into the pan and fry until golden brown. Add the dumplings to the soup shortly before the end of the cooking time. Season to taste with salt and pepper.
Pluck the chervil leaves from the stems and sprinkle over the soup.