Wash the potatoes, cut in half and cook in salted water for about 15 minutes. Finely dice the onion. Heat the oil in a small pot, fry the onion in it. Add 75 ml water and 50 ml milk, stir in broth and bring to the boil. Stir in mustard.
Add vegetable binding agent while stirring and simmer for about 2 minutes. Bring approx. 500 ml salt water and vinegar to the boil. Beat the egg into a ladle and carefully slide it into the vinegar water. Cook over low heat for 56 minutes. Take out with a skimmer, drain. Wash the chives, dab dry, chop finely. Drain potatoes, mash coarsely. Add 3 tablespoons of milk. Season with salt and nutmeg. Fold chives into the puree.
Wash the chives, dab dry, chop finely. Drain potatoes, mash coarsely. Add 3 tablespoons of milk. Season with salt and nutmeg. Fold chives into the puree. Arrange mashed potatoes, poached egg and mustard sauce on a plate. Garnish with tomato wedge, lettuce leaf, chives and parsley