Peel and dice the onion. Cut the sausages into slices. Fry in hot oil, remove. Sauté the onion in the frying fat, dust the curry over it and sauté briefly. Stir in 3/4 l water, stock and tomatoes with their juice. Season to taste with salt, pepper and sugar. Cover and simmer for 10 minutes.
Puree the soup and stir in the ketchup. Heat the sausage in it. Season the soup to taste.
Serve with sour cream and sticks. Dust with curry.