Preheat oven to 400°F. Place cauliflower on a baking sheet lined with aluminum foil. Drizzle 1 tablespoon olive oil over each head of cauliflower. Set aside. Trim ¼-inch from top of garlic heads to expose tops of garlic cloves, leaving cloves intact. Pour ½ tablespoon olive oil over each garlic head and wrap in aluminum foil. Place on prepared pan with cauliflower. Roast 30 minutes or until cauliflower edges start to brown. Remove cauliflower from pan, cool slightly and cut into small florets. Set aside. Return garlic to oven and roast an additional 15–20 minutes. Remove from oven and let rest 15 minutes. Gently unwrap garlic heads and squeeze each garlic head to remove roasted cloves from their skins. Set cloves aside, discarding peels. In a large Dutch oven, melt butter over medium-high heat. Add onions, celery and carrots then cook 5–7 minutes or until vegetables are wilted. Add roasted cauliflower and roasted garlic then cook 3–5 minutes, stirring frequently. Add flour, stirring to incorporate ingredients. Gradually add stock and thyme leaves, bring to a boil then reduce to simmer. Cook 20–25 minutes or until cauliflower is tender. Using a hand-held blender, purée soup until smooth. Add heavy whipping cream and season to taste using salt, pepper and granulated garlic. Garnish with chopped parsley and serve hot.