Peel the zucchini and potatoes and cut into small cubes. Peel onion and garlic and chop finely. Heat the oil, let the onion take a nice color in it, then add water, dissolve the stock cube in it and add the vegetables. Season with caraway seeds, salt, pepper and the garlic, bring to the boil and simmer for about 15 minutes - then puree (either puree everything through or only half - we like it when there are still pieces of vegetables in it).
Mix the egg yolk with the sour cream and stir into the soup, bring to the boil once, remove from the plate, season to taste and serve sprinkled with chives.