Preheat a large non-stick skillet over medium heat for about 2-3 minutes.

Spread 1 tablespoon of unsalted butter on one side of each slice of sourdough bread.

Place a slice of buttered bread, butter-side down, in the skillet. Layer with thin slices of roast beef.

Add a layer of sharp cheddar cheese on top of the roast beef. Optionally, spread Dijon mustard on the unbuttered side of another slice of bread.

Place another slice of buttered bread on top of the sandwich, buttered side up.

Cook for about 3-5 minutes or until the bottom is golden brown, then flip the sandwich and cook for another 3-5 minutes.

Once both sides are crispy and the cheese is melted, remove from skillet and let cool for a minute.

Carefully cut the sandwich into halves or quarters and serve hot.