1. Prepare Your Panini Press: Begin by preheating your panini press according to the manufacturer’s instructions. If you don’t have a panini press, you can use a large skillet or griddle over medium heat, along with a heavy pan or brick wrapped in foil to press the sandwiches.

2. Butter the Bread Slices: Generously spread softened butter on one side of each sourdough bread slice. This buttered side will be the exterior of your panini, ensuring a beautiful golden-brown and crispy finish.

3. Assemble the Sandwiches: On the unbuttered side of four slices of bread, spread a thin layer of Dijon mustard if using. Layer half of the cheddar cheese, followed by a generous amount of thinly sliced apples. If adding turkey, place it on top of the apples. Finish with the remaining cheddar cheese, then top with the other four slices of bread, buttered side up.

4. Press the Paninis: Carefully place one or two assembled paninis onto the preheated panini press, depending on its size. Close the lid and press firmly.

5. Cook Until Golden and Melty: Cook for 4-7 minutes, or until the bread is deeply golden brown and crispy, and the cheese is completely melted and gooey. The exact time will depend on your panini press and desired crispiness.

6. Remove and Rest: Once cooked, carefully remove the paninis from the press. Let them rest on a cutting board for a minute or two before slicing. This allows the cheese to set slightly and prevents it from oozing out too much when cut.

7. Slice and Serve: Slice each panini in half, either straight across or diagonally, and serve immediately while warm.