1. In a large saucepan, heat oil over medium-high heat. Add onion and leeks, stir well, reduce heat to medium, and cover. Gently cook, stirring once or twice, until onions start to become soft and translucent, about 10 minutes.
2. Add potato, cauliflower, and broth. Broth should just barely cover the vegetables - if not enough, add water to cover. Raise heat to high and bring to a boil; then reduce to medium and simmer, partially covered for 25 to 30 minutes, or until vegetables are tender.
3. Puree soup using a hand-hel immersion blender. Alternatively, let soup cool for 10 minutes, and then transfer vegetables and broth in batches to a blender, puree until smooth, and return to pan.
4. Stir in buttermilk, salt, and pepper, and mix until blended. Reheat until hot if necessary. To serve, ladle into bowls and sprinkle each serving with 2 Tbsp. chopped watercress and a few grindings of black pepper.
Source: Hannaford fresh Magazine, September - October 2008