Butter both sides of the bread slices; then spread the grainy Dijon mustard on one side of each slice. Layer the cheese slices on top of the mustard side of each slice of bread, and then layer the apple slices on top of the cheese slices on one of the slices of bread. Take the slice without the apples and flip it over on top of the apple slice so that the apple slices are sandwiches between the cheese slices. Cook on a very lightly greased skillet or wide frying pan over medium-high heat until the bread slices are a toasty brown and the cheese is melted, about 4–6 minutes on each side. Serve immediately.
“I loved the textural crunch from the apple slices, and the mustard grains played nicely against the smoky creaminess of the melted cheddar.” - Eva Kosmas, Adventures in Cooking