1. Prepare your bread slices: Lay out all eight slices of sourdough bread on a clean surface. Lightly butter one side of each slice with the softened unsalted butter. This buttered side will be the exterior of your sandwich, ensuring a beautiful golden-brown crust.

2. Assemble the first layer: Take four slices of bread and, on the unbuttered side, spread a thin layer of mayonnaise and/or Dijon mustard if using. This adds an extra layer of flavor and moisture.

3. Layer the turkey: Divide the deli turkey breast evenly among these four slices of bread, arranging it neatly so it covers most of the surface. A slight overlap is fine.

4. Add the cheese: Place one slice of provolone cheese on top of the turkey on each of the four bread slices. If you want extra cheesiness, you can add another half slice.

5. Complete the sandwich: Place the remaining four unbuttered sourdough slices on top of the cheese, with the buttered side facing outwards. Gently press down on each sandwich to compact the layers slightly.

6. Cook the sandwiches: Heat a large non-stick skillet or griddle over medium heat. Once hot, carefully place the sandwiches in the skillet, buttered side down. You may need to cook them in batches depending on the size of your pan.

7. Grill to golden perfection: Cook for 4-6 minutes per side, or until the bread is golden brown and crispy, and the cheese is thoroughly melted and gooey. You can gently press down on the sandwiches with a spatula while they cook to ensure even browning and good contact with the pan.

8. Serve immediately: Once cooked, remove the sandwiches from the skillet. For easier eating, you can slice them in half diagonally. Serve hot and enjoy the warm, cheesy goodness!