Preheat the oven to 425°. In a large bowl, toss garlic, cauliflower and 2 tablespoons of oil; spread on a rimmed baking pan and roast for 25 minutes or until golden brown, stirring once halfway through roasting.
In a large saucepot, heat remaining 1 tablespoon oil over medium-high heat. Add celery and onion; cook and stir for 5 minutes or until tender. Add thyme; cook and stir for 1 minute or until fragrant. Stir in stock, lemon juice, 1/2 teaspoon salt, 1/4 teaspoon pepper and cauliflower mixture; heat to a simmer. Reduce heat to low; cover, cook and stir for 20 minutes.
With a blender or stick blender, purée soup until smooth. Makes about 8 cups.
Serve soup garnished with pepper and/or green onions, if desired.
Approximate nutritional values per serving (1 cup): 92 Calories, 5g Fat, 1g Saturated Fat, 0mg Cholesterol, 194mg Sodium, 9g Carbohydrates, 3g Fiber, 4g Sugars, 0g Added Sugars, 3g Chef Tip(s):If the soup is too thick to blend smooth, add up to 1 cup of water.