In a large pot or Dutch oven, melt 2 tablespoons of butter over medium-high heat. Add half of the sliced mushrooms and cook without stirring for 3-4 minutes until deeply browned. Stir occasionally and cook for another 3-5 minutes until all the moisture has evaporated and the mushrooms are fully caramelized. Remove the browned mushrooms and set aside for garnish.

Reduce heat to medium. Add the remaining 2 tablespoons of butter to the pot. Add the minced shallot and cook until softened, about 3 minutes. Add the minced garlic and dried thyme, cooking until fragrant (about 1 minute).

Add the remaining mushrooms (uncooked) to the pot. Sauté until they soften and reduce in volume, about 7-10 minutes. Season lightly with salt and pepper.

Pour in the chicken broth. Bring the mixture to a simmer and cook for 10 minutes to allow the flavors to meld. If using, carefully sprinkle the xanthan gum over the simmering liquid while whisking constantly to prevent clumping. Let it simmer for 1 minute to see if desired thickness is achieved.

Stir in the heavy cream. Heat through until the soup is hot, but do not allow it to boil rapidly after adding the cream. Taste and adjust salt and pepper as necessary.

For a smoother texture, use an immersion blender to partially blend the soup directly in the pot (leaving some texture is traditional). Alternatively, blend half the soup in batches in a standing blender and return to the pot. Serve hot, garnished generously with the caramelized mushrooms reserved earlier and fresh parsley.