Assemble the sandwiches! Spread ½ tablespoon butter one of each slice of bread and sprinkle generously with flaky sea salt (this adds major flavor to the sandwich!). On the opposite side of each slice of bread, spread ½ tablespoon of mustard. Layer the sliced apple on two slices of bread and top with a piece of cheese. Add the arugula to the other two slices of bread, top with a slice of cheese and the smoked turkey. Gently flip the slices of bread with turkey over onto the slices of bread with the apple, forming sandwiches.