Wash potatoes well and cook with skin in boiling water for about 20 minutes.
Boil the eggs hard for about 10 minutes. Wash herbs, shake dry, pluck off leaves and chop finely. Cut cress from the bed. Mix herbs with yoghurt, cream cheese, milk and mustard. Quench eggs, peel, chop finely and stir in. Season sauce with salt and pepper.
Drain the potatoes and let them evaporate briefly. Serve with roast beef and sauce.