1. In a medium rondeau melt the butter (no color) and gently sweat the onions, garlic, and leeks until tender, again no color or browning.
2. Add the potatoes and the chicken stock. Bring to boil and simmer until the potatoes are fully cooked. Puree in a large blender until completely smooth and silky. Add the cream and pulse until fully blended.
3. Take 3/4 cup of the soup and return to the blender. Add the watercress and puree until completely smooth. Swirl into the soup as a garnish.