1. Prepare your ingredients: Start by thinly slicing your apple and cheddar cheese. Aim for slices that are uniform in thickness so they cook evenly. If using shredded cheese, ensure it’s evenly distributed.

2. Butter the bread: Spread a thin layer of softened butter evenly on one side of each slice of sourdough bread. This buttered side will be the exterior of your panini, ensuring a golden, crispy crust.

3. Assemble the first sandwich: Place two slices of bread, butter-side down, on a clean surface. If using, spread a very thin layer of Dijon mustard on the unbuttered side of one slice.

4. Layer the fillings: On the mustard-coated slice (or one of the unbuttered slices), layer half of the cheddar cheese, followed by half of the apple slices. Top with the remaining half of the cheddar cheese. Place the other slice of bread, unbuttered side down, on top to complete the sandwich. Repeat for the second panini.

5. Preheat your panini press or skillet: If using a panini press, preheat it according to the manufacturer’s instructions. If using a skillet, heat it over medium heat.

6. Cook the panini: Carefully place the assembled panini into the preheated panini press and close the lid, pressing down gently. Cook for 4-5 minutes, or until the bread is golden brown and the cheese is melted and bubbly. If using a skillet, place the panini in the hot skillet, then place another heavy skillet or a foil-wrapped brick on top to press it down. Cook for 4-5 minutes per side, or until golden brown and cheese is melted.

7. Serve immediately: Once cooked, carefully remove the panini from the press or skillet. Let it rest for a minute or two before slicing in half and serving warm.