1. Preheat oven to 400°.
2. Combine cauliflower and 2 teaspoons oil in a bowl, tossing gently to coat. Spread cauliflower in a 15 x 10-inch jelly-roll pan coated with cooking spray. Bake at 400° for 30 minutes or until browned, stirring occasionally.
3. Heat remaining 1 teaspoon oil in a Dutch oven over medium heat. Add leek, celery, and garlic; sauté 5 minutes or until tender. Add roasted cauliflower and thyme; sauté 1 minute. Add broth, salt, and pepper; bring to a boil. Cover, reduce heat, and simmer 5 minutes. Remove from heat; cool 15 minutes.
4. Place half of cauliflower mixture in a blender or food processor; process until smooth. Return purée to pan. Combine milk and flour in a small bowl. Stir with a whisk; add to pan. Bring to a simmer, and cook 3 minutes or until thickened, stirring constantly with a whisk.