Cut the zucchini into approx. 1cm thick slices. Peel the potatoes and chop them like zucchini. Cut the onions into small cubes. Melt the butter in a sufficiently large soup pot and let the onion turn golden yellow in it. Now add the zucchini, potatoes and the vegetable stock, cook for about 30 minutes. Now add the sour cream and puree everything with the blender. Salt and pepper to taste and you`re done. To garnish, cut a piece of zucchini raw into small cubes and sprinkle over the soup to serve.