Put a large stockpot or dutch oven over a medium flame and add the butter. Chop onion and add to the melting butter. Stir occasionally as the onion become translucent.Add the pumpkin puree and all other ingredients (except cream and xanthan gum) then stir. Bring to a boil and then simmer for 10 minutes.
Using an emulsion blender or a free standing blender, blend the soup to thoroughly puree all the onions into a velvety smooth and creamy soup. You can pass through a mesh sieve to get the soup even smoother, but not necessary.