Spread each bread slice evenly with mustard. Arrange half of cheese and all of the bacon and apple slices on 2 bread slices. Top with remaining cheese and bread slices, and press together gently.
Melt butter in large cast-iron or nonstick skillet over medium-high heat. Add sandwiches to pan, and cook, pressing down with back of spatula, 3 to 5 minutes on each side or until cheese melts and bread is golden brown.