Peel, wash and quarter the potatoes. Peel and finely chop the onion. Melt the fat. Brown onion and potatoes in it. Deglaze with broth, season with salt and pepper.
Cover and cook for about 20 minutes. Clean and wash the watercress and drain well. Add to the soup and puree. Refine with white wine and cream, season again. Serve portion by portion garnished with fresh cress and chilli threads