• Melt the butter over medium low heat and gently cook the onions and leeks until soft. This could take as long as 10 minutes. You do not want anything to brown though. Add the diced potatoes and chicken stock and simmer until the potatoes are tender, around 15-20 minutes.
• Let cool slightly and puree. You want to really puree - no lumps here. That's why waxy or even yukon gold are high risk. Pureeing waxy potatoes is a good way to make glue. Not good.
• One little trick to be aware of. If you are pureeing in a blender you need to take the little cap in the middle of the top out to let the steam escape. Not doing this is a pretty good way to get soup on the ceiling.
• At this point you can either finish it off or just let it sit until dinner time. If you let it sit, reheat to the barest simmer to serve. In either case you will not let this come to a simmer again after you add the cream so plan accordingly.
• Add cream to the hot soup and then season with the truffle salt/oil and white pepper. You want to add the truffle salt just before you serve. I find the flavour weakens if you cook it.
• Ladle into bowls, garnish with a bit of minced chives and serve.

Variation

• If you can get your hands on fresh sorrel this is a nice flashy variation. Sweat some sorrel in butter. Follow the recipe above but omit the truffle salt.

  Puree the wilted sorrel in a bit of soup to make a sorrel puree. Ladle the soup into the bowl and then spoon some of the sorrel puree into the middle of the soup.

  Cut through the circle with a knife to drag the green sorrel puree through the white leek and potato soup. Prepare for adulation from your dinner guests and serve.