01 - Preheat a large skillet or griddle over medium heat.
02 - In a separate pan, cook the beef bacon over medium heat until crisp, approximately 3 to 4 minutes per side. Drain on paper towels.
03 - Butter one side of each bread slice evenly.
04 - Place 4 bread slices buttered side down on a work surface. Layer each with 2 slices of cheese, 2 slices of beef bacon, and optional tomato or onion. Spread a thin layer of Dijon mustard if desired.
05 - Top each sandwich with remaining bread slices, buttered side up.
06 - Place sandwiches in the preheated skillet. Grill until golden brown and cheese is melted, approximately 3 to 4 minutes per side. Press gently with a spatula for even browning.
07 - Remove from skillet, slice in half, and serve warm.