Brown bacon in a large saucepan over medium-high heat, stirring occasionally, until just crisp, about 4 minutes. Spoon off all but 1 tablespoon of the fat. Add onion and celery; sauté until vegetables are soft, about 4 minutes.
Add stock, cauliflower, chicken, salt, pepper, thyme and sage. Bring to a boil; reduce heat to medium, partially cover and simmer until vegetables are just tender, about 5 minutes. Whisk in thickener and simmer until soup thickens slightly, about 2 minutes. Stir in cream and simmer to heat through before serving.