HEAT oven to 425°F.
ARRANGE 4 rows of zucchini alternately with 3 rows of tomatoes on bottom of 9-inch square baking dish sprayed with cooking spray, overlapping rows as necessary. Sprinkle with onions; drizzle with dressing. Cover.
BAKE 30 to 35 min. or until zucchini is tender, uncovering after 20 min.
TOP with basil and cheese; bake 5 min. or until cheese is melted.