Cut Bread Into 1-In.-Thick Slices, Leaving Slices Attached At Bottom. In A Bowl, Combine The Butter, Onions And Mustard; Spread On Both Sides Of Each Slice Of Bread. Cut Each Cheese Slice Diagonally Into Four Triangles; Place Between The Slices Of Bread. Cut Bacon In Half Widthwise And Then Lengthwise; Drape A Piece Over Each Slice. Place The Loaf On A Double Thickness Of Heavy-Duty Foil. Bake At 400