Add the chicken breasts to the slow cooker in a single layer.
Next, add the celery, carrot, onion, and garlic.
Then, add the chicken broth, wild rice, thyme, and bay leaves.
Cook on high for 4 hours or low for 6 hours.
Then, remove the chicken and shred or slice it. Return the chicken to the slow cooker along with the mushrooms, heavy cream, and butter. Stir.
Cook for an additional 45 minutes on high or 1.5 hours on low.
Enjoy warm and store any leftover soup in an airtight container in the fridge for up to three days.