2. Add the potatoes and cook for 2 minutes without coloring.
3. Add the vegetables and cook for 4 minutes without coloring.
4. Add the nage and horseradish, bring to the boil and simmer for 5 minutes and set aside.
5. Heat a clean pan, add the remainder of the oil, and gently cook the spinach and watercress in a hot pan until the watercress's bitterness is balanced.
6. Add this to the liquid mix, blend until smooth and very green, then pass through a conical strainer.