Wash the apple and slice it into thin pieces about β
-inch thick. Slice or shred the cheddar cheese so it melts evenly during cooking.
Place cheddar cheese on one slice of bread, add a layer of apple slices, then add another layer of cheese. Cover with the second slice of bread.
Spread softened butter evenly on the outside of both bread slices, covering the edges for a crisp golden crust.
Heat a skillet over medium-low heat. Cook the sandwich for 3β5 minutes on each side until the bread is golden and the cheese is melted.
Remove the sandwich from the pan and let it rest for 2 minutes before slicing. Cut diagonally and serve warm.