Preheat broiler. Line a baking sheet with foil.
Halve the baguette lengthwise, and then halve crosswise to make four 5-inch long pieces. Place bread on the baking sheet. Broil, 2 to 4 minutes or until lightly toasted.
Spread each piece of baguette with 1 tsp. mustard. Top with apple slices, chicken, and cheese. Broil sandwiches, 2 to 3 minutes more, or until cheese is melted.