Peel onions and cut them into fine pieces. Heat the butter in a saucepan and sauté the onions until translucent.
Clean and quarter the mushrooms. Add to the onion and cook for 8 minutes. Dust with flour and sweat it briefly. Pour in the vegetable stock and bring to the boil. Add the cream and stir in the mustard. Season with salt and pepper and simmer for 10 minutes. In the meantime, wash the chives and cut into fine rolls.
Peel the eggs and quarter them lengthways. Season the sauce to taste and season to taste. Carefully mix in the eggs and chives.