Dice the onions and fry them in the margarine until golden. Dust with flour and deglaze with the stock and stock. Finely dice the soup vegetables and tomatoes and add both. Season with ketchup and salt and simmer for 5 minutes.
Now add the washed and roughly diced fish fillet and let it simmer for a few minutes on a low flame. Just before serving, stir in the mustard and add the capers. Stir in the finely chopped herbs and distribute the soup on serving plates. Toast the white bread, cut out round shapes, place on top of the hot soup and serve with a dollop of sour cream on each.