In A Small Bowl, Combine The Mozzarella Cheese, Mushrooms, Mayonnaise, Parmesan Cheese And Onion. Cut Bread In Half Lengthwise; Spread Cheese Mixture Over Cut Sides. Grill, Covered, Over Indirect Heat Or Broil 4 In. From The Heat For 5-10 Minutes Or Until Lightly Browned. Slice And Serve Warm.