Peel and wash the potatoes and cook them in boiling salted water for about 20 minutes. Boil eggs for about 8 minutes until soft. Peel onion and chop finely. Melt butter and sauté onion.
Add stock and bring to the boil. Mix yoghurt with flour and stir into the stock, simmer for 2-3 minutes. Stir in mustard and simmer for about 5 minutes at low heat. Wash the chives and cut into small rolls.
Drain eggs, quench, peel and halve. Drain the potatoes. Stir chives into the sauce and season with salt and pepper. Arrange potatoes, eggs and sauce on plates. Garnish with radishes.