For the watercress soup, first sort out the watercress, removing the large stems. Rinse watercress and set aside about a quarter of the bunch for garnish.
Clean the spring onions and cut the whites to 3-4 cm from the greens into narrow rings.
Rinse and peel the potato and cut into 1 cm cubes.
Melt the butter in the saucepan and sweat the spring onions a tiny bit over medium heat. Add the cress and fall together for another 4-5 minutes.
Deglaze with the clear soup and pour in the potatoes. Cook everything together for about 10 minutes, then stir in crème fraîche and bring to a boil.
Season with salt, pepper and a pinch of nutmeg, then blend the watercress soup with a blender. Decorate with the remaining cress leaves and bring to the table.