Find 2 heavy skillets that will nest together. 2 10" cast iron skillets are ideal. Don't use non-stick; it's not safe to heat a non-stick skillet to these temperatures when empty. Place both skillets over high heat.
Spread mustard on one slice of bread. Distribute the cheese evenly over the mustard, season with a twist of freshly ground black pepper and top with the second slice of bread (which you may spread with mustard or not, as you choose). Lightly spritz the upward-facing bread surface with olive oil, using a mister.
When the pans are hot enough to vigorously sizzle a drop of water, remove them from the heat and place the sandwich, oiled side down, in the middle of one pan. Lightly spritz the side now facing you, as well as the bottom of the second pan. Place the second pan on top of the sandwich. If the top pan isn't cast iron, weight it down with a brick or heavy can.
When you hear the first drop of cheese run out of the sandwich and sizzle on the pan, it's done. This will take 3-5 minutes. Remove to a plate, count to 10 (to let the cheese set up ever so slightly), cut in half and serve.