Total Time: 45 Minutes
Makes: 4 servings
1 large bunch watercress (about 6 ounces)
2 tablespoons unsalted butter
1/4 cup finely chopped onion
1 pound Yukon gold potatoes, peeled and cut into 1/3-inch cubes
2 cups vegetable broth
1/3 cup heavy cream
Coarse sea salt
Set aside 4 nice sprigs watercress for garnish; trim and roughly chop remaining watercress.
In a medium pot, melt butter over medium-low heat. Add onion; cook, stirring occasionally, until softened, about 5 minutes. Add potatoes and broth; bring to a gentle simmer and cook for 10 minutes. Add chopped watercress and 1 ⁄ 4 cup water; stir until watercress is wilted (liquid in pot should just cover vegetables; add more water, if necessary), then continue to gently simmer until potatoes are very tender, 15 to 20 minutes more.
In a blender, carefully purée soup in batches. Return soup to pot, stir in cream and 1/3 cup water (add more water, if desired, to thin soup to your liking) and gently heat to warm through. Season with salt to taste.
Ladle soup into warm bowls. Garnish with reserved watercress.