- Spread mustard on two slices of bread, then divide cheese slices between them. Top each with a remaining bread slice, and lightly butter the outside of both sandwiches.
- Heat large nonstick skillet over medium heat. Place one sandwich in pan, and heat until cheese begins to soften and bread turns golden brown. Flip and heat until other side is golden and cheese is melted. Repeat with second sandwich.
- Open each sandwich enough to insert half of tomato and arugula, if using. Serve hot.