Peel, wash and cut the potatoes into pieces. Cover the potatoes and cook them in little boiling salted water for about 20 minutes.
Peel and finely chop the onion. Heat the fat in a pot, fry the onions until transparent, dust with flour and sauté briefly. Add stock and milk while stirring and bring to the boil. Simmer at low heat while stirring for about 5 minutes.
Boil the eggs in boiling water for 8-9 minutes until soft. Wash parsley, shake dry and chop. Stir mustard into the sauce and season with salt, pepper and sugar.
Drain the potatoes and let them evaporate. Rinse eggs under cold water, peel and halve. Arrange boiled potatoes, eggs and mustard sauce on plates. Sprinkle with parsley. Serve with a colourful leaf salad.