01 - Preheat a large skillet or griddle over medium heat, ensuring it's ready for grilling.
02 - In a separate pan, cook the beef bacon over medium heat until it becomes crisp, approximately 3 to 4 minutes per side. Transfer to paper towels to drain excess fat.
03 - Butter one side of each of the 8 bread slices evenly.
04 - Place 4 bread slices, buttered side down, on your work surface. Layer each with 2 slices of cheese, followed by 2 slices of cooked beef bacon. If desired, add thinly sliced tomato, red onion, and a thin layer of Dijon mustard.
05 - Cover each assembled sandwich with the remaining bread slices, ensuring the buttered side faces upwards.
06 - Carefully place the prepared sandwiches into the preheated skillet. Grill until each side is golden brown and the cheese is thoroughly melted, which typically takes about 3 to 4 minutes per side. Gently press down with a spatula to ensure even browning and contact.
07 - Remove the finished sandwiches from the skillet, slice them in half, and serve immediately while warm.