Cut Bread Lengthwise Down The Middle To Within 1/2 In. Of Bottom. Cut Diagonally Into 1-In. Slices To Within 1/2 In. Of Bottom. Repeat Cuts In The Opposite Direction. Stuff Bread With Cheese And Onions. Place On A Double Thickness Of Heavy-Duty Foil (About 24 In. X 12 In.). Combine The Butter, Lemon Juice And Mustard; Drizzle Over Bread. Fold Foil Around Bread. Place On A Baking Sheet. Bake At 425