Cook pasta according to package directions, omitting salt and fat. Add broccoli and 1 cup cauliflower during last 2 minutes of cooking; drain.
Place flour in a large saucepan. Gradually add stock, stirring constantly with a whisk until blended. Bring to a boil over medium heat; cook 2 minutes or until thick, stirring constantly. Add remaining 2 cups cauliflower and garlic. Bring to a boil; cook 15 minutes or until very tender. Place cauliflower mixture in a blender. Add milk, butter, pepper, juice, salt, and 1 ounce cheese. 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); process until smooth. Return cauliflower mixture to pan over low heat. Add pasta mixture and lemon rind; toss well to coat noodles. Sprinkle with remaining 1 ounce cheese. Serve immediately.