Heat oven to 450°F. Line 15x10x1-inch pan with foil. In 10-inch nonstick skillet, cook ground beef and onion over medium-high heat, stirring frequently, until beef is thoroughly cooked; drain. Stir in Worcestershire, mushrooms and 1 cup cheese.
Spoon beef mixture evenly onto baguette halves, spreading mixture to edges. Cut each half into 3 sections. Place sandwiches in pan; cover with additional foil.
Bake 15 minutes. Remove cover; top with 1/2 cup cheese. Bake 5 minutes longer or until cheese is melted.