Preheat a large soup pot over medium-high heat with 2 turns of the pan of oil. Add the sliced knockwurst and cook until lightly browned, about 4-5 minutes.
Remove the browned knockwurst to a plate and add the caraway seeds to the pot. Toast for abut 30 seconds then add the onion, bay leaf and red cabbage, season with a little salt and pepper and cook, stirring frequently for 5 minutes or until the onions and cabbage start to get tender -- put a lid on it to get it really going.
Add the sugar and apple cider vinegar, cook for about 1 minute, then add the chicken stock and tomato sauce. Bring up to a bubble and simmer the soup for about 10 minutes. Add the knockwurst back into the soup and simmer another minute just to heat it up again.