MIX Mustard, butter, Parmesan, basil and garlic powder in a large bowl until well blended. Add zucchini, squash and onions; toss to coat well.
DIVIDE vegetable mixture evenly among sheets of aluminum foil. Bring up sides of foil; double fold top and ends to tightly seal each packet.
GRILL packets over medium-high heat 10 to 12 minutes or until zucchini is tender. Sprinkle with additional Parmesan cheese before serving, if desired.