POTATO-WATERCRESS SOUP WITH PESTO

Steps:

• To make the soup, cook potatoes in boiling water until tender, about 30 minutes. When cool enough to handle, slip off the skins and pass through a ricer. Set aside.
• Bring a large pot of water to a boil. Add 8 cups of the watercress and blanch for 5 seconds. Immediately drain and refresh under cold running water. Drain and squeeze out excess water. Place in a blender with the yogurt and blend until smooth. Scrape into a bowl and whisk in the milk, mustard, 2 teaspoons salt and pepper to taste. Coarsely chop the remaining 2 cups watercress and stir into the soup. Refrigerate until chilled.
• Meanwhile, to make the pesto, blanch the watercress as above. Drain, squeeze out excess water and coarsely chop with a knife. Place in a blender with the walnuts, garlic, olive oil and salt. Blend until smooth, stopping as necessary to scrape down the sides of the jar.
• Taste soup and add more salt if needed. Ladle the soup among 4 bowls. Place 2 teaspoons of the pesto in the center of each bowl and sprinkle the mustard seeds around it. Serve immediately.