Heat the butter in a heavy bottom pot over medium heat. Add in the onion and cook until soft. Should take about 5-8 minutes.
Add in the garlic and the chopped mushrooms and continue to cook for 5 more minutes.
Add in the chicken broth, heavy cream and sprinkle the xanthan gum over the top and whisk in.
Bring to a boil for a few minutes to let thicken.
If you want a completely smooth soup, you can carefully blend it in a food processor, or you can blend half of the soup if you want it to have a bit of texture. You could also use an immersion blender directly in the pot, just be careful of the hot soup!
Season with salt and pepper to taste.