First make a marinade with the vinegar, water, mustard and oil.
Peel the onions and cut into fine rings. Peel the sausage, cut it into slices and then into strips. Add to the marinade with the onions and season with salt and pepper. Chill the salad for about 30 minutes and let it steep. Before serving, season again with pepper and possibly salt.
Serve as a side dish with brown bread with butter.