How to Make It
Cook bacon in a Dutch oven over medium, stirring often, until crisp, 5 to 7 minutes. Using a slotted spoon, transfer bacon to paper towels to drain. Reserve 1 tablespoon drippings in pan; discard remaining drippings.
Increase heat to medium-high. Add leek, celery, and garlic to hot drippings in pan; cook, stirring often, until crisp-tender, about 5 minutes. Add cauliflower, stock, salt, and pepper; bring to a boil. Cover and reduce heat to medium. Simmer until cauliflower is very tender, about 15 minutes.
Remove 1 cup vegetables with a slotted spoon; finely chop.
Pour remaining mixture into a blender; add half-and-half. Remove center piece of blender lid (to allow steam to escape); attach lid, and place a clean towel over opening in lid. Process, starting slowly and increasing speed, until very smooth, 1 minute and 30 seconds to 2 minutes. Return mixture to pan with chopped vegetables; cook over medium until warmed through, about 2 minutes. Ladle soup into bowls; top with bacon, cheese, and chives.
Slow Cooker Method - Prepare as directed in step 1, substituting a skillet for Dutch oven. Omit step 2. Combine drippings, leek, celery, garlic, cauliflower, stock, salt, and pepper in a 6-quart slow cooker. Cover and cook on LOW until tender, about 4 hours. Proceed with steps 3 and 4.