I made this watercress soup recipe today, substituting nasturtium leaves (without stems, about 4 loosely packed cups) for the watercress. The cookbook (Chez Panisse Vegetables) also suggests using spinach. I had it with some croutons on top, and gave up on the sieve because none of the vegetable was ending up in the soup itself. I have to remember to puree this in the blender, not with the hand blender, because it's too fibrous for the hand blender to handle.
2 bunches watercress (~1 lb)
1 yellow onion
1 clove garlic
2 Tbsp. olive oil
4 cups chicken stock (I used vegetable) or water
A few parsley leaves
A few tarragon leaves
Salt and pepper
Creme fraiche (I didn't use this)
Pick through the watercress and discard any thick stems.
Peel and slice the onion and garlic thin and stew them in the olive oil, covered, until soft and translucent. Add the stock, bring to a simmer, and cook, uncovered, for about 10 minutes. After 5 minutes, add the parsley. Have ready a large bowl half filled with ice and a smaller, stainless steel bowl that will nest inside it and rest on the ice.
Remove the soup from the heat, add the watercress and tarragon, and allow the soup to stand for 5 minutes, no longer. Immediately puree the soup in a blender and pour it through a medium-fine sieve into the bowl on ice. Stir the soup until it is at room temperature, then remove it from the ice and season to taste with salt and pepper.
Reheat the soup to a simmer just before serving; do not boil. Ladle the soup into bowls and garnish lightly with lines of lightly salted creme fraiche streaked on the surface.