Place bread slices on a flat surface and spread butter on one side of each slice, making sure to reach all edges.

Flip the slices and spread mayonnaise evenly on the inside. Add mustard if using.

Place turkey slices evenly on one piece of bread, allowing slight folds for texture.

Add cheese on top of the turkey and sprinkle salt and black pepper.

Close the sandwich with the second slice, buttered side facing out.

Heat a pan on medium heat until warm.

Place the sandwich on the pan and cook for 3 to 4 minutes until the bottom becomes golden brown.

Press gently with a spatula and flip carefully.

Cook the other side for another 3 to 4 minutes until crispy and the cheese is melted.

Remove from the pan and let it rest for 1 minute before slicing.

Cut and serve warm.