Wash and peel your onion and potato then roughly chop for 5 seconds / speed 5. Add both to the mixing bowl along with the celery.
Add the butter to the mixing bowl then sauté the vegetables for 5 minutes.
Add the stock with the measuring cup in place then sauté for 20 minutes / 230°F|110°C.
Soak the watercress and spinach in water then pick the leaves off the stems.
Add the spinach and watercress to the mixing bowl then sauté for 10 minutes / 230°F|110°C.
Add cream, white pepper and salt then mix for 2 minutes / speed 10. Make sure the measuring cup is in place. Serve immediately to avoid discolor of the soup or immediately cool in ice bath to make the soup in advance.