1. Prepare your bread and spread: Lay out your four slices of sourdough bread. On one side of each slice, spread a thin layer of softened butter. This buttered side will be the exterior of your panini, ensuring a golden, crispy crust.

2. Apply the mustard (optional): On the unbuttered side of two of the bread slices, spread a thin layer of Dijon mustard. This adds a subtle tang that complements the other flavors beautifully.

3. Layer the turkey: Place two slices of cooked turkey breast on top of the mustard-spread bread slices (or directly on the unbuttered side if omitting mustard). Ensure the turkey is evenly distributed.

4. Add the cheese: Distribute half of the sharp cheddar cheese over the turkey on each sandwich. The cheese will melt and bind all the ingredients together.

5. Arrange the apple slices: Neatly layer the thinly sliced apple over the cheese. Try to keep the apple slices in a single layer for even cooking and easier pressing.

6. Top with more cheese and pepper: Place the remaining cheddar cheese over the apple slices. Sprinkle a pinch of black pepper over the cheese to enhance the savory notes.

7. Assemble the sandwich: Place the remaining two slices of sourdough bread (buttered side up) on top of the apple and cheese layers, completing your sandwiches.

8. Cook the panini: Preheat your panini press according to the manufacturer’s instructions. If you don’t have a panini press, you can use a large skillet over medium heat, placing a heavy pan on top of the sandwich to press it down. Cook for 5-7 minutes, or until the bread is golden brown and crispy, and the cheese is thoroughly melted and gooey.

9. Serve immediately: Carefully remove the paninis from the press or skillet. Let them cool for just a minute or two before slicing diagonally and serving hot.