cheeses like Parmigiano-Reggiano, Asiago, Pecorino, aged Manchego, and aged Gouda MAKES 2 SANDWICHES 2 tablespoons unsalted butter 4 slices high-quality white, whole wheat, or rye sandwich bread 4 ounces sliced Category A cheese (see Note above) Kosher salt ½ ounce category B cheese, grated (optional; see Note above) Brown mustard Melt ½ tablespoon of the butter in a 12-inch stainless steel or cast-iron skillet over medium heat Add bread slices and swirl them around the pan with your hands until all the butter is absorbed Cook, swirling the bread occasionally, until very lightly browned, about 1 minute Remove to a cutting board, toasted side up, and immediately top with the sliced A cheese Melt another ½ tablespoon butter in the skillet and toast the remaining 2 bread slices until lightly browned, then immediately press them toasted side down onto the cheese-topped slices to form sandwiches Melt another ½ tablespoon butter in the skillet and sprinkle with a pinch of salt Reduce the heat to mediumlow and place the sandwiches in the skillet Swirl them around with your hands until all the butter is absorbed Cook the sandwiches, swirling them with your hands and pressing down on them gently with a wide stiff spatula occasionally, until the bottoms are a deep, even golden brown, about 4 minutes Remove the sandwiches to your cutting board with the spatula Melt the remaining ½ tablespoon butter in the skillet, sprinkle with a bit of salt, and repeat the toasting procedure with the second side, until the sandwiches are golden brown on both sides and the cheese is thoroughly melted, about 4 minutes longer Transfer the sandwiches to the cutting board If desired, spread the grated cheese evenly over a large plate Press the sandwiches into the grated cheese, turning once and pressing on them until you get an even coating of cheese on both sides Return the sandwiches to the skillet and cook until the grated cheese has melted and formed a golden brown crust, about minute Carefully flip the sandwiches and repeat with the second side Transfer to the cutting board Slice the sandwiches in half on the diagonal, and serve immediately, with mustard and the tomato soup ... repeat the toasting procedure with the second side, until the sandwiches are golden brown on both sides and the cheese is thoroughly melted, about 4 minutes longer Transfer the sandwiches to the cutting board...occasionally, until the bottoms are a deep, even golden brown, about 4 minutes Remove the sandwiches to your cutting board with the spatula Melt the remaining ½ tablespoon butter in the skillet, sprinkle with a bit of salt,... spread the grated cheese evenly over a large plate Press the sandwiches into the grated cheese, turning once and pressing on them until you get an even coating of cheese on both sides Return the sandwiches to