**Prepare the Bread:** Lay out the sourdough slices. Spread a thin layer of butter on one side of each slice. If using mustard or mayonnaise, spread it on the other side.
**Assemble the Sandwich:** On the unbuttered side of one slice, layer cheddar cheese and turkey slices. Top with another slice of bread, buttered side facing out. Repeat for the second sandwich.
**Heat the Skillet:** Preheat a large skillet or griddle over medium heat.
**Cook the Sandwiches:** Place the sandwiches in the skillet, buttered side down. Cook for 3–4 minutes, pressing gently with a spatula, until the bread is golden brown and crispy. Flip and cook the other side for another 3–4 minutes, until the cheese is melted and the bread is golden.
**Serve:** Remove from the skillet, slice in half, and serve warm.