Wash potatoes thoroughly and cook in boiling water for about 20 minutes. Boil 2 eggs hard in boiling water for about 10 minutes, rinse and peel. Mix kefir, quark, sour cream and mustard.
Wash the herbs, shake dry and chop coarsely. Cut the cress from the bed and, except for something to garnish, mix with the herbs and add to the quark mixture. Finely chop with a hand blender and season to taste with vinegar, salt, pepper and sugar.
Coarsely chop the boiled eggs and stir into the sauce. Boil the rest of the eggs in boiling water for about 6 minutes until they are waxy, rinse and peel them. Drain the potatoes. Arrange eggs, sauce and potatoes on plates and sprinkle with cress.