1. Prepare Your Ingredients: Start by thinly slicing your apple to about 1/8-inch thick. If using, thinly slice your cheddar cheese or have your shredded cheese ready. Soften your butter.

2. Butter the Bread Slices: Generously spread softened butter on one side of each slice of sourdough bread. This buttered side will be the exterior of your panini.

3. Assemble the Panini: Place two slices of bread, butter-side down, on a clean surface. If using, spread a thin layer of Dijon mustard on the unbuttered side of these two slices. Layer half of the cheddar cheese on each of these slices. Arrange the apple slices evenly over the cheese, then top with the remaining cheddar cheese. Place the other two slices of sourdough bread on top, butter-side up.

4. Preheat Your Panini Press or Skillet: Turn on your panini press and allow it to preheat fully. If you don’t have a panini press, heat a large skillet or griddle over medium heat.

5. Cook in a Panini Press: Carefully place one or two assembled paninis into the preheated panini press. Close the lid and press firmly. Cook for 4-7 minutes, or until the bread is golden brown and crispy, and the cheese is completely melted.

6. Cook in a Skillet (No Panini Press): If using a skillet, place the assembled paninis into the hot skillet. Place a heavy object (like another skillet or a foil-wrapped brick) on top to press them down. Cook for 3-5 minutes per side, or until the bread is golden brown and the cheese is melted. Flip once to brown both sides evenly.

7. Serve Immediately: Once cooked, carefully remove the panini from the press or skillet. Let it rest for a minute or two before slicing diagonally. Serve warm and enjoy!