Spread the bottom half of bread slices with mustard. Then, layer evenly with sliced cheese, ham and apples. Top each sandwich with remaining bread slices. Lightly butter both outer halves of each sandwich.
Heat a large nonstick skillet over medium heat. Cook sandwiches, turning once, until bread is golden, approximately 5 minutes on each side. Transfer sandwiches to a baking sheet lined with tin foil. Bake in oven until cheese is melted, approximately 8-10 minutes.
Remove from oven. Cut each sandwich diagonally in half; serve warm.