Place cauliflower, onion, celery and garlic into a 5 or 6-qt slow cooker. Stir in chicken broth, thyme, bay leaf and 2 cups water; season with salt and pepper, to taste.
Cover and cook on low heat for 3-4 hours, or until vegetables are tender. Open crockpot and remove bay leaf.
Puree mixture carefully with an immersion blender or transfer to regular blender and puree until smooth. Return back to crockpot if necessary.
Stir in cheese and half and half.
Serve immediately, garnished with additional cheese, bacon and chives, if desired.