01 -
Mayo goes on the outer sides of both bread slices while mustard spreads inside. Then stack the cheese on the mustard side.
02 -
Turn the heat to medium-low. Let some butter melt and spread evenly in the pan.
03 -
Place the sandwich in the pan. Cover it, and leave for 2-3 minutes until it turns golden.
04 -
Turn it over, cover again, and cook 1-3 minutes till browned and gooey cheese. Lower the heat if things move too quickly.