Halve the bell pepper, remove the stalk base, seeds and inner ribs. Wash halves and then cut into strips. Peel the skin off the sausage, cut the sausage into strips. Peel the onions and cut into rings. Mix vinegar, mustard, salt, pepper and oil, mix with the salad ingredients and let steep for at least 15 minutes. Season to taste with salt, pepper and a little sugar before serving.