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.
Heat remaining 1 teaspoon oil in a Dutch oven over medium heat.
Add leek, celery, and garlic; saut 5 minutes or until tender.
Add roasted cauliflower and thyme; saut 1 minute.
Add broth, salt, and pepper; bring to a boil. Cover, reduce heat, and simmer 5 minutes.
Remove from heat; cool 15 minutes.
Place half of cauliflower mixture in a blender or food processor; process until smooth. Return pure 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.