Peel the onion and chop it finely, fry the onion cubes with butter (2 tablespoons) in a high pan until translucent. Dust the flour (2 tbsp) over it and let it sweat. Deglaze / pour on with water (500 ml) and milk 500 ml). Add stock cube (10 g) and bring to the boil, stirring constantly. Season with salt (1 teaspoon), pepper (½ teaspoon) and sugar (1 teaspoon). Finally fold in / add the mustard (3 tbsp), the cream (100 ml) and the boiled eggs.