1. Lengthwise cut each squash into 3/8-inch-thick slices. Cut eggplant into 12 slices. Place squash, eggplant and onion slices in large resealable plastic bag. Drizzle with soy sauce, orange juice and olive oil. Seal bag. Place in shallow pan. Refrigerate for 1 to 12 hours, turning occasionally. Drain vegetables. Discard marinade.
2. Coat cold grill rack with nonstick coating. Preheat grill. Place burgers and onion slices on grill rack over medium heat. Grill for 3 minutes. Add eggplant slices to grill rack. Grill for 3 minutes more. Add squash slices to grill rack. Continue grilling for 5 to 6 minutes or until vegetables are tender and burgers are heated through, turning once.
3. Cut burgers into strips. Separate onion slices into rings. Cut eggplant slices into strips. Top tortillas with burger strips, onion rings, eggplant strips, lettuce and cheese. Roll-up. Horizontally cut each in half.
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