01 - Cook the beef bacon in a skillet over medium heat until crisp, approximately 5 to 7 minutes. Drain excess fat using paper towels.
02 - Lightly toast all bread slices. Optionally, spread a thin layer of butter on each slice for enhanced flavor.
03 - Evenly spread mayonnaise, and Dijon mustard if using, on one side of each toasted bread slice.
04 - Place one slice of bread mayo-side up on a plate. Layer with a romaine leaf, turkey slices, and a tomato slice. Season lightly with salt and pepper. Add a second bread slice mayo-side down, spread mayo on top side, then layer with beef bacon and optional red onion slices. Top with the final bread slice mayo-side down.
05 - Gently press the sandwich, then cut into halves or quarters using a serrated knife. Optionally, secure each portion with toothpicks.
06 - Serve immediately, optionally accompanied by pickles or chips.