Look for watercress with small, bright green leaves and thin stems to make this peppery cold soup.
Remove and discard root end and dark green top of leek. Cut in half lengthwise, and rinse under cold running water to remove grit and sand. Thinly slice leek.
Melt butter in a Dutch oven over medium heat; add leek, and sauté 8 to 10 minutes or until tender. Add garlic, and sauté 1 minute.
Add broth and potato; cover and cook, stirring occasionally, 20 minutes or until potato is tender. Remove from heat, and add spinach and next 4 ingredients, stirring until spinach is wilted.
Puree soup with a blender until smooth; pour into a large bowl. Whisk in buttermilk and lemon juice; add salt and pepper to taste. Serve chilled or at room temperature.