Peel and dice the onion and sauté in hot fat until translucent. Dust with flour, sauté briefly and deglaze with the vegetable stock. Cook for 5 minutes while stirring.
Boil the eggs for 8 minutes, quench and peel them.
Cut the watercress leaves from the stems and add to the sauce except for a few leaves. Puree with the cutting stick. Season the sauce with salt, pepper, lemon juice and a pinch of sugar. Whip the cream until it is half stiff and stir in with the whisk.
Halve the eggs and arrange on top of the sauce with a few cress leaves.
This goes well with mashed potatoes with fried onions.