No panini press? Cook sandwiches in a skillet 4 minutes on each side with a pan on top.
Spread about 1 tsp. mustard on 1 side of each bread slice. Place 1 cheese slice on each of 4 bread slices; top with apples. Top with remaining cheese and bread, mustard sides down. Coat both sides of sandwiches with cooking spray. Cook sandwiches in a preheated panini press 8 to 10 minutes or until golden brown.