1. Preheat oven to 425°.
2. Remove white papery skin from garlic head (do not peel or separate the cloves). Wrap in foil. Arrange cauliflower in a single layer on a baking sheet coated with cooking spray. Add garlic to pan. Bake garlic and cauliflower at 425° for 35 minutes or until golden brown, turning cauliflower after 20 minutes. Cool 10 minutes. Separate garlic cloves; squeeze to extract garlic pulp. Discard skins.
3. Heat olive oil in a large Dutch oven over medium-high heat. Add onion to pan; saute 8 minutes or until tender, stirring frequently. Add milk, broth, and thyme sprigs; bring to a simmer. Cook 10 minutes, stirring occasionally.
4. Discard thyme sprigs. Stir in reserved garlic pulp and cauliflower. Place half of milk mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl. Repeat procedure with remaining milk mixture. Stir in hazelnut oil, salt, and pepper. Ladle 1 cup soup into each of 12 bowls; top each serving with 2 teaspoons parsley.