Cook bacon in heavy saucepan over medium heat until golden and some of the fat has been released. Add onion, celery and garlic. Cover and cook until vegetables are soft, stirring occasionally, about 7 minutes. Add cauliflower, chicken stock and Parmesan cheese. Bring to the boil. Reduce heat, cover and simmer until cauliflower is tender, about 20 minutes.
Puree soup in a blender or food processor. Add milk and warm through before serving.