Remove the stems from the radish leaves, wash thoroughly and drain. Melt the butter in a saucepan and sweat the radish leaves in it.
Peel the potatoes, cut into small cubes and add to the radish leaves with the finely diced onion. Top up with the hot vegetable stock and season with salt, pepper and nutmeg to taste. Cook for about 20 minutes over medium heat.
Take the soup off the stove and puree with the hand blender. Fold in the whipped cream. Serve sprinkled with finely chopped pimpinelle.