Spread mustard over 4 slices of bread.
Top each slice of bread with 1 slice cheese, 2 bacon slices, ¼ apple slices, 1 slice of cheese and remaining slices of bread.
Spray a preheated Panini grill with cooking spray. Place sandwiches on grill; spray tops of sandwiches with additional cooking spray. Grill until bread is golden brown and cheese is melted (about 5 minutes).