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