Peel, wash and cut the potatoes into pieces. Cover and cook in boiling salted water for about 20 minutes.
For the sauce, peel and finely dice the onion. Heat 2 tablespoons of butter in a pot and fry the onion until transparent. Dust with flour and sauté briefly. Add 1⁄4 l water and 1⁄4 l milk while stirring, stir in stock, bring everything to the boil.
Simmer at low heat while stirring for about 5 minutes. Stir mustard into the sauce, season with salt, pepper and sugar.
In the meantime, boil the eggs for about 6 minutes until they are soft. Drain the potatoes. Add 100 ml milk and 2 tablespoons butter. Mash with a potato masher to a creamy puree. Season to taste with salt and nutmeg.
Quench the eggs, peel them. Arrange mashed potatoes, eggs, mustard sauce and gherkins. Garnish with parsley as desired.