Peel, wash and chop the potatoes and boil them in salted water for about 25 minutes. Put eggs in boiling water and boil for 9-10 minutes. Clean the salad, wash and shake dry.
Stir the yoghurt and 2-3 tablespoons of milk until smooth and season to taste with a little sugar, lemon juice and salt. Melt the fat in a small pot. Add flour and sweat for 1-2 minutes. Stir in 250 ml milk, add stock and simmer for about 1 minute while stirring.
Season the sauce with salt, pepper and mustard. Wash the chives, shake dry and cut half into rings. Add half of the rings to the sauce. Drain the eggs, quench and peel. Drain potatoes.
Arrange eggs, potatoes and sauce, sprinkle with chives and garnish with chive stems. Arrange salad leaves on plates, add yoghurt sauce.