Preheat a griddle or skillet over medium heat.
Butter one side of each slice of sourdough bread.
Spread a thin layer of Dijon mustard on the unbuttered side of one slice.
Layer one slice of cheddar cheese, cooked bacon, and apple slices on the mustard side.
Top with the other slice of cheddar cheese, then place the other slice of bread on top, buttered side facing outward.
Place the sandwich on the griddle and cook for 3-4 minutes per side, or until the bread is golden brown and the cheese is melted.
Slice the sandwich in half and serve immediately.
