Cut off the cress and put something aside for decoration.
Finely dice the onion, garlic and celeriac. Fry in hot butter, dust with flour and deglaze with white wine and stock. Let simmer for 20 minutes.
Add the cream and cress. Puree and then bring to the boil again briefly. Season to taste with salt, pepper and nutmeg. Serve sprinkled with the rest of the cress.