01 - Cook the beef bacon in a skillet over medium heat until crisp, about 5 to 7 minutes. Drain on paper towels.
02 - Lightly toast the bread slices. If desired, spread a thin layer of butter on each slice for additional flavor.
03 - Spread mayonnaise evenly over one side of each slice of toasted bread. Add Dijon mustard if using.
04 - Place one slice of bread mayo side up on a plate. Top with a lettuce leaf, turkey slices, and a tomato slice. Season lightly with salt and pepper. Add a second slice of bread mayo side down, then spread mayo on the top side. Layer with beef bacon and red onion if desired. Finish with the final slice of bread mayo side down.
05 - Gently press the sandwich, then cut into halves or quarters using a serrated knife. Secure with toothpicks if preferred. Serve immediately.