Wash the potatoes and cook in boiling water for about 20 minutes. Drain potatoes, rinse with cold water and peel, let cool. Mix stock with 350 ml hot water. Fold in 1 tablespoon of mustard, 3 tablespoons of oil and vinegar.
Season with salt, pepper and sugar. Cut the potatoes into thick slices. Mix with the vinaigrette. Soak toast in water. Peel and finely chop the onion. Squeeze the toast. Knead with minced meat, onion, 1 teaspoon mustard and egg.
Season with salt and pepper. Form small balls. Heat 2 tablespoons of oil and fry meatballs for 5-7 minutes, turning them over. Remove and put aside. Clean and wash spring onions and cut into rings.
Clean, wash and quarter the radishes. Clean, wash and drain lettuce and watercress. Pluck the lettuce into small pieces. Cut cress from the bed. Mix the meatballs and other prepared ingredients into the potatoes.
Season everything once more.