Combine first 4 ingredients in a small microwave-safe dish; microwave at HIGH 30 seconds. Stir in onion and ketchup; microwave at HIGH 1 minute. Place cauliflower in a 2-quart casserole coated with cooking spray. Spoon onion mixture over cauliflower; toss well to coat. Cover and microwave at HIGH 9 minutes, turning after 4 minutes.