In a small bowl, mix mayonnaise and mustard.
Spread a thin layer of mayo mixture on one side of each slice of bread.
There will be mixture leftover.
Place one slice of bread on skillet over medium heat, mayo side down.
Top with cheeses and second piece of prepared bread, mayo side out.
Toast until golden brown on the bottom, then flip.
Toast the second side until golden brown and the cheese is melted.
Remove from skillet and enjoy!