5 medium leeks (white and pale green parts only), chopped (4 cups)
1 1/2 Tbs. vegetable oil
In large pot, heat oil over medium heat. Add leeks and onion and cook until tender, stirring occasionally, about 10 minutes. Add potato and watercress. Cook until wilted, stirring occasionally, 5 minutes. Add stock, bring to a boil. Reduce heat and simmer until potato is tender, about 30 minutes. Stir in Worcestershire sauce and season with salt and pepper.
In food processor or blender, purée soup in batches. Serve warm or chilled garnished with chopped chives if desired.
Can be made the day before or frozen for up to 1 month.