Heat butter in a heavy bottom pot over medium heat. Add onions and cook until soft, about 5–8 minutes.
Add garlic and chopped mushrooms and continue to cook for 5 more minutes. Add chicken broth, heavy cream and sprinkle 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.
• I like to use a food processor to chop the mushrooms finely
• For the mushroom topping, I saved a couple of mushrooms and sliced them thin, then cooked them until crisp in a little oil.
Nutrition per serving: Calories: 281k cal, Carbohydrates: 6g, Protein: 9g, Fat: 22g, Saturated Fat: 14g, Fiber: 1g, Sugar: 4g