- 3 Tbsp unsalted butter
- 2 cups white or yellow onion, chopped
- 1 cup white wine, chicken stock, or vegetable stock (wine or veg stock for vegetarian version)
- 450g potatoes, peeled and cut into chunks
- 6 cups water
- 6 cups fresh watercress, about 200g, chopped, stems included
- 1/2 teaspoon black pepper
- About 6 Tbsp sour cream, stirred in, or for garnish
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.
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.
Add the watercress to the pot. Stir well and cook for 2 minutes. 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.
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.