1. Preheat oven to 400°.
2. Place cauliflower in a 2-quart broiler-safe baking dish lightly coated with cooking spray; coat cauliflower with cooking spray. Sprinkle with 1/4 teaspoon salt; toss. Bake at 400° for 30 minutes or until almost tender. Cool 5 minutes.
3. Preheat broiler.
4. Melt butter in a saucepan over medium heat. Remove from heat. Stir in panko. Stir in 1/4 cup cheese and chives.
5. Heat a medium saucepan over medium-high heat. Coat pan with cooking spray. Add onion to pan; sauté 4 minutes or until almost tender, stirring frequently. Add garlic; sauté 1 minute, stirring constantly. Add flour; cook 1 minute, stirring constantly. Gradually add milk, stirring with a whisk; bring to a boil. Cook 3 minutes or until thick, stirring constantly. Remove from heat; stir in remaining 1/4 cup cheese, remaining 1/4 teaspoon salt, parsley, and pepper. Pour milk mixture over cauliflower mixture; toss. Top evenly with cheese mixture. Broil 3 minutes or until golden brown and thoroughly heated.