In a small bowl, combine Dijon mustard and honey, stirring until smooth and well-blended. Set this mixture aside.

Lay out four slices of whole grain bread and spread the honey-Dijon mixture on one side of each slice.

On top of the honey-Dijon side of two slices, layer sliced turkey, cheddar cheese, and thinly sliced Granny Smith apples. Sprinkle with salt and pepper.

Place the remaining two slices on top, honey-Dijon side facing down, pressing gently to compact the sandwich.

Spread softened unsalted butter generously on the outer sides of your assembled panini.

Preheat your panini press or skillet over medium heat and grill until golden brown and cheese is melted.

Let the panini cool for a minute, slice diagonally, and serve warm.