01 -
Heat cast-iron skillet on low. Cut bread into 1/2-inch slices and grate both cheeses
02 -
Spread 1 teaspoon mayonnaise on one side of each bread slice
03 -
Melt butter in skillet, add bread mayo-side down, pile on cheese, top with second slice mayo-side up
04 -
Cover and cook 2-3 minutes until bottom is golden brown
05 -
Flip sandwich, cover and cook 2 minutes until cheese melts and second side browns
06 -
Cut diagonally and serve immediately while cheese is melty