Cook cauliflower with water in 2 quart covered casserole 6 minutes on high or until barely tender.
Remove cauliflower and place zucchini, onion, butter in casserole.
Heat, uncovered, on high for 2 minutes on high or until vegetables are tender.
Add cauliflower, cheese, and tomatoes to zucchini mixture, mix in salt and thyme.
Heat covered on high 4 minutes or until tomatoes are tender.