Spread softened butter on one side of each bread slice, all the way to the edges.
Flip one slice butter-side down, spread mustard on the inside if using, and layer on the cheese. Top with the second slice, butter-side up.
Heat a nonstick skillet over medium-low heat.
Place the sandwich in the pan and cover with a lid. Cook for 3 minutes.
Remove the lid, flip the sandwich, and cover again. Cook for another 2 to 3 minutes, until golden and fully melted.
Let the sandwich sit for 30 seconds before cutting.