Watercress and arugula soup has to be one of the easiest, lightest, tastiest soups you can make with these two super greens. The flavor is peppery and refreshing and the color out of this world, plus it's delicious eaten hot or chilled. It's delicious.
Take the watercress and separate the thick, tough stems from the leaves. Reserve the stems and set the leaves aside. Finely dice the stems
Over a medium high flame, heat the oil in a 5 qt Dutch oven or other heavy pot. Add the diced watercress stems and the Vidalia onion and turn down the heat to medium. Sprinkle with salt and sautée until the watercress stems have softened and the onion is soft and transparent, about 5-8 minutes. Take care not to brown the onion.
Add the stock. Raise the heat to medium high and bring to a boil. When the soup is boiling, lower the flame to medium and simmer for 15 minutes.
Add the reserved watercress leaves and the arugula. Cook until they have just wilt and have turned a vivid bright green. Turn off the heat. Liquidize the soup in batches in a high speed blender and return to the pan. Taste for salt. Serve hot or chilled with a dollop of creme fraiche or yogurt, some chopped chives and some homemade croutons.
If you can find the peppery wild Italian arugula, it’s an added plus. Also you can substitute olive oil for the ghee or butter. If you make this with vegetable stock or broth, make sure it is light in texture and taste.