Clean and dice the onion. Cut the fennel into small to medium-sized cubes. Keep the fennel green as a decoration.
Sauté the onion and fennel in a saucepan with melted butter. Deglaze everything with the vegetable stock and cook for 20 minutes. Then chop up with the hand blender and add the whipped cream. Season to taste with pepper from the mill.
Arrange the soup on plates and serve with a spoon of crème fraiche and the fennel greens.