Roasting cauliflower brings out a delicious nutty flavor and gives the soup a gorgeous silky texture. Prepare the soup through Step 4 up to two days ahead. Let cool; cover and chill. Reheat just before you're ready to serve. Serve the soup as a seated starter while your turkey rests.
Preheat oven to 425°. Cut cauliflower into 2-inch florets; toss with olive oil and 1/2 tsp. salt. Arrange florets in a single layer on a jelly-roll pan. Wrap garlic cloves in aluminum foil, and place on jelly-roll pan with cauliflower.
Bake at 425° for 30 to 40 minutes or until cauliflower is golden brown, tossing cauliflower every 15 minutes.
Transfer cauliflower to a large Dutch oven. Unwrap garlic, and cool 5 minutes. Peel garlic, and add to cauliflower. Add stock, and bring to a simmer over medium heat; simmer, stirring occasionally, 5 minutes. Let mixture cool 10 minutes.
Process cauliflower mixture, in batches, in a blender until smooth, stopping to scrape down sides as needed.
Return cauliflower mixture to Dutch oven; stir in milk, cheese, and remaining 1 tsp. salt. Cook over low heat, stirring occasionally, 2 to 3 minutes or until thoroughly heated. Add pepper to taste.