To prepare the soup, peel and finely dice the onion. Heat the oil in a large pot and fry the onion until transparent. Add tomato puree and sauté briefly while stirring. Add tomatoes and juice and puree finely.
After the fondue, pour the fondue stock into the soup. Bring to the boil covered and then simmer open for about 40 minutes.
Stir vodka into the soup to serve. Season to taste with salt, pepper, tabasco and sugar. Stir the sour cream until creamy. Serve with the soup.