Preheat a panini press or place a skillet over medium-low heat.
Lay four slices of bread on a flat surface. Top each with ½ tablespoon of mustard, followed by one slice of Swiss cheese. Add 2 ounces of smoked turkey, followed by three to four apple slices. Top with remaining cheese and bread.
Butter the top of each sandwich and place butter side down on the hot panini press. Butter the remaining side, then close the press and grill for 3 to 5 minutes, or until bread turns golden brown and cheese melts.
If using a skillet, cook for 3 to 4 minutes per side, or until bread turns golden brown and cheese melts.
Serve immediately, while still warm and melty!