PREHEAT oven to 400°F. Scoop out center of each bread half, leaving 1-inch-thick shells.
COOK and stir mushrooms, onions and garlic in hot oil in skillet until tender. Spoon into bottom of bread loaf. Sprinkle with cheese; cover with top of bread. Wrap in foil.
BAKE 15 to 20 min. or until cheese is melted. Cut crosswise into 12 slices to serve.