In a large stockpot melt the butter over medium-high heat. Add in mushroom and stir to coat. Stir the mushrooms and sprinkle with a pinch of salt. The mushrooms will start to weep and release their moisture.
Continue to stir the mushrooms about 10 minutes and they will start to brown and caramelize. Once the mushrooms have cooked down add in the chopped onion and stir to coat. Continue cooking the mushroom and onion mixture until the onions start to soften.
Add in the chicken stock, garlic cloves, water, sherry, thyme, 1/2 tsp salt and bring to a quick boil. Then turn the heat to very low and cover your pot to simmer for 1 hour.
After the hour of simmering remove the lid and carefully puree your mushroom mixture. I use an immersion blender, but you can transfer to a blender to puree in small batches. If using a stand alone blender you will then transfer your soup back into a clean pan.
Once the soup is fully blended stir in the heavy cream until all combined. Last step is to sprinkle the xanthan gum like pixie dust over the soup while whisking.
Season salt and pepper to taste.
Notes
**An immersion blender makes the pureeing of the soup so easy, but you can use a blender....just be careful with hot liquid.**A note on the nutrition calculations...this soup makes about 60 oz of soup. The serving size is for 10 small cups of soup to be enjoyed as a side to your meal.