Spread 1 1/2 Tbsp. butter on 1 side of 4 slices of bread, and spread mustard on other side of each slice. Top mustard covered side of each bread slice with 1 slice Cheddar, followed by 1/4 each of turkey and apple slices. Spread remaining butter on 1 side of remaining 4 slices of bread and place them, butter side up, on top of apple slices.
Place sandwiches in a large skillet and cook over medium-high heat until golden, about 3 to 5 minutes per side. If skillet is too small, cook sandwiches 2 at a time, placing first batch on a plate and covering loosely with foil while cooking second batch. Serve immediately.