Combine cauliflower pieces and onion in a 2-quart casserole.
Cover with plastic wrap.
Microwave at HIGH 5 to 6 minutes or until tender-crisp.
Let stand 3 or 4 minutes.
Combine cauliflower, onion, tomatoes, and salad dressing in the 2-quart casserole.
Toss well to coat with dressing.
Sprinkle cheese over tossed vegetables.