Wash and core the tomatoes and cut into fine wedges. Peel the onions and slice them into rings. Cut the cucumber and the peeled sausage into thin slices. Rinse the chives, pat dry and chop.
Whisk together vinegar, mustard, salt, pepper and 2 tablespoons of water. Withhold the oil. Add to the salad ingredients as a dressing. Mix and serve on lettuce leaves if necessary.