Watercress can often be found, when in season, in the herb section of the grocery store. The bunches should be thick-stemmed, and the leaves should have a strong, peppery bite to them. The delicate, thin-stemmed baby watercress that some markets carry is not appropriate for this soup.
- 3 tablespoons butter
- 2 cups white or yellow onion, chopped
- 1 cup white wine, chicken stock, or vegetable stock (wine or veg stock for vegetarian version)
- 1 pound potatoes, peeled and cut into chunks
- 6 cups water
- 6 cups fresh watercress, about 1/2 pound, chopped, stems included
- 1/2 teaspoon black pepper
- About 6 tablespoons sour cream, stirred in, or for garnish
1 Cook the onions: In a large pot, heat the butter until frothy, then cook the onions over medium heat until soft and translucent, about 5 minutes. Salt the onions as they cook.
2 Add potatoes, wine/stock, simmer: Add the wine or stock, potatoes and water and bring to a boil. Turn the heat down to low, cover the pot and simmer until the potatoes are soft, about 30 minutes.
3 Add watercress: Add the watercress to the pot. Stir well and cook for 2 minutes.
4 Purée the soup: Turn off the heat and purée the soup with an immersion blender. If you don't have an immersion blender, pour the soup in batches into a blender and transfer the blended soup to a clean pot. Be careful when blending hot liquids to only fill the bowl a third of the way, and to hold down the blender's lid while you purée the soup.
5 Season and garnish to serve: Add salt to taste, then add the black pepper. You can either stir the sour cream into the whole batch of soup, or serve a tablespoon in the center of each person's bowl.