Heat the butter in a wide saucepan and sauté the mushrooms for 5 minutes, until golden brown. Season with a pinch of salt and pepper before adding half of the celery stalks and keep cooking for 1-2 minutes.
Add the cauliflower rice and stir. Season with salt, garlic powder, and pepper and sauté for 2-3 minutes until fragrant.
Add the broth one ladle at a time, stirring until the cauliflower is tender and the consistency is creamy and thick.