Mix mayonnaise, vinegar, mustard and tarragon in a small bowl, add 2-3 tsp. water.
Boil green beans in salted boiling water and drain in a colander, pour over with cold water. Cut the onion into half rings, cucumbers and tomatoes into circles. Cut the eggs into 6-8 slices. Cut the cheese into thick strips. Place the salad mixture, vegetables, cheese and eggs in a salad bowl, pour dressing over and toss gently.