Saute onion and butter in a soup pot over medium heat for 3-5 minutes or until softened. Add minced garlic during the last minute.
Add broth, broccoli, celery, salt, and pepper. Bring to a boil over high heat, then reduce heat to medium low and simmer, uncovered, for 8-10 minutes or until broccoli is tender.
Stir in the cream and heat through. Add the cheese a half a cup at a time, stirring until melted between each addition.
If desired, quickly stir in xanthan gum until soup thickens.