How to Make It
Melt butter in a large Dutch oven over medium. Add onion, thyme, salt, and garlic. Cover and cook, stirring occasionally, until onion is very soft and slightly caramelized, 10 to 12 minutes. Add flour to Dutch oven, and cook, stirring constantly, 1 minute. Add stock and potato. Increase heat to high, cover, and bring to a boil. Add cauliflower, and reduce heat to medium. Cook, uncovered, until cauliflower is tender, about 20 minutes.
Transfer cauliflower mixture to a blender; add cayenne. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening, and process until smooth, about 1 minute. Ladle soup into bowls. Top with bacon and chives.