01 - Arrange all bread slices on a clean cutting board.
02 - Spread 1 tablespoon mayonnaise evenly over one side of each bread slice; this will form the crust.
03 - Place 2 slices of cheddar cheese each onto the uncoated side of two bread slices. Add Dijon mustard, tomato slices, and freshly ground black pepper if using.
04 - Close the sandwiches with the remaining bread slices, ensuring the mayo-coated side faces outward.
05 - Heat a nonstick skillet or griddle over medium-low. Transfer sandwiches to the pan and cook for 3 to 4 minutes per side, pressing gently with a spatula, until deeply golden and cheese is fully melted. Adjust temperature to avoid burning.
06 - Remove sandwiches from skillet, let rest for 1 minute, slice, and serve immediately.