1) Heat the butter in a soup pot over medium-high heat and cook the onions until soft, about 5 minutes. Do not brown them. Add the garlic and sauté for 1 minute. Sprinkle with salt.
2) Add the jerusalem artichokes and the chicken stock to the pot and bring to a simmer. Reduce the heat to low, and simmer, covered, until the jerusalem artichokes begin to break down.
3) Using an immersion blender or upright blender, purée the soup.
4) Add the cream and simmer for another 10 minutes.
For some extra special flavour, add truffle oil.