Easy Watercress and Leek Soup

Bring stock, leeks, potato, and garlic to a boil in a medium pot. Reduce heat to medium; cook until leeks are tender and potato is soft, about 15 minutes. Add chopped watercress; cook until beginning to soften, about 2 minutes.

Puree the stock and vegetables in a blender until smooth (work in batches, if necessary, to avoid filling blender more than halfway). Season with salt and pepper. Top each serving with sour cream, and garnish with watercress, if desired.