Place two 10-inch cast-iron skillets over high heat for 5 minutes.
Meanwhile, spread the mustard on one slice of bread. Distribute the cheese evenly over the mustard, season with pepper, and top with the second slice of bread. Brush the bread surface with the clarified butter. Go for even coverage, but don’t soak the bread or it won’t brown.
When the pans are hot, kill the heat and place the sandwich, top-side down, in the middle of skillet number 1. Brush the newly exposed bread face with butter and lay the bottom of skillet number 2 right down on top of the sandwich. Wait. In 3 to 5 minutes, you will hear the sizzle of melted cheese oozing out into the pan. (If you don’t, you went skimpy with the cheese.)
Gently remove the top skillet and use a flat metal spatula to remove the sandwich to a waiting cutting board and wait 60 seconds. You will be hungry, so at this time you may want to examine skillet number 1 for toasty little bits of cheese, which you can hungrily devour. When 60 seconds is up, you may wish to cut the sandwich in half with a serrated knife. It won’t make the sandwich taste any better, but there’s something about biting into that cut side that’s very satisfying.