- 2 tablespoons unsalted butter
- 2 leeks, halved and thinly sliced (white and light green parts only)
- kosher salt
- 1 small russet potato, chopped
- 2 cups low-sodium chicken broth
- 2 bunches watercress, thick stems discarded (about 6 cups)
- 1 tablespoon fresh tarragon leaves, plus more for serving
- 1/2 cup buttermilk
- Melt the butter in a medium pot over medium heat. Add the leeks and ½ teaspoon salt. Cook, stirring occasionally, until tender, 4 to 6 minutes.
- Add the potato, broth, and 4 cups water and bring to a boil. Reduce heat and simmer until the potato is tender, 8 to 10 minutes. Add the watercress and tarragon. Cook until the watercress is bright green, about 1 minute.
- Working in batches, puree the soup in a blender. Add the buttermilk and ½ teaspoon salt and pulse to combine. Serve topped with tarragon leaves.