Find 2 heavy skillets that will nest together. Two (10-inch) cast iron skillets are ideal. Heat them over high heat.
Meanwhile, spread mustard on one slice of bread. Distribute the cheese evenly over the mustard, season with fresh black pepper and top with second piece of bread.
Spritz the bread surface that's staring up at you with olive oil using either a Misto or a pump sprayer. A light coat will do, don't soak.
When the pans are hot enough to vigorously sizzle a drop of water, remove them from the heat and place the sandwich, top-side down in the middle of one pan. (if your pans are a different size, this would be the smaller one.) Spritz the slice now facing you, as well as the bottom of the other skillet. Lay the skillet right on top of the sandwich. If the top pan isn't cast iron, weigh it down with a brick, can, or something of similar heft.
Wait patiently, crack a beer. When you hear the first bit of cheese run out and sizzle on the pan, it's done. This will take anywhere from 3 to 5 minutes.
Carefully remove the top skillet, (you may need to coax it off with a spatula, but I doubt it). Just look at it. It's perfect ... better than mom's. (no reason to tell her)
Remove to a plate, count to 10 and slice it in half. Take a bite. Take another. So they lost ... there's always next year.