In a soup pot, heat the oil over medium-high heat. Add the knockwurst to the pot and cook until lightly browned all over, 3-4 minutes. Set aside on a plate.
Add the caraway seeds to the pot and toast for 30 seconds, then add the cabbage, onion and bay leaf and cover and cook until the vegetables begin to soften, about 5 minutes. Add the cider vinegar and sugar and cook for 1 minute. Stir in the chicken broth and tomato sauce and bring to a boil, then lower the heat and simmer for 10 minutes. Return the knockwurst to the stoup to heat through. Season to taste with salt and pepper.
In a small bowl, mix the apple, lemon juice and parsley. Serve the stoup in shallow bowls and dollop spoonfuls of the apple mixture on top. Pass the bread and butter around the table.