Cut a thin slice off bottom of zucchini so it sits flat. Cut a thin lengthwise slice from top of zucchini and discard. Scoop out pulp, leaving a 1/4-in. shell. Chop 1 cup pulp; set aside. Place zucchini shells in a ungreased 8-in. square microwave-safe dish. Cover and microwave on high for 3 minutes or until crisp-tender; drain and set aside.
In a microwave-safe bowl, combine mushrooms, onion and reserved zucchini pulp. Cover and microwave on high for 4 minutes; drain. Heat 1-2 minutes longer; drain thoroughly. Stir in the corn, butter, thyme, basil, salt and pepper. Cover and microwave on high for 1 minute. Stir in salad dressing and 1/4 cup cheese. Stuff into zucchini boats.
Microwave, uncovered, on high for 2-3 minutes. Sprinkle with the remaining cheese; top with tomatoes. heat 1-2 minutes longer or until the vegetables are tender and the cheese is melted. Let stand for 3-5 minutes before serving.