Rinse the potatoes and cook in boiling salt water for 20-25 minutes. Rinse the green beans and cut into small pieces. Blanch the green beans in boiling salt water for 8-10 minutes.
Drain the potatoes and green beans. Rinse the green beans in cold water. Peel the potatoes.
Cut the potatoes into small pieces. Bring the broth to a boil and pour over the potatoes. Rinse the radishes and chop. Rinse and drain the lettuce.
Mix the yogurt, mustard, mayonnaise and milk. Season with salt, pepper and lemon juice. Mix the potatoes, beans, radishes and the yogurt mixture. Loosely fold in the lettuce and serve immediately.