01 - Cook turkey bacon in skillet over medium heat until crispy, approximately 4-5 minutes per side. Transfer to paper towel-lined plate to drain excess grease.
02 - Toast all bread slices until golden brown. While warm, spread softened butter lightly on each slice if desired for extra flavor.
03 - Spread mayonnaise evenly on one side of each toasted bread slice. Add Dijon mustard to mayo for extra tang if preferred.
04 - Place first bread slice mayo-side up. Layer with half the lettuce leaves, half the turkey breast slices, and tomato slices. Season with salt and pepper.
05 - Place second bread slice mayo-side down onto stack. Spread mayo on top side. Add half the cooked turkey bacon, additional lettuce, tomato slices, and onion if using.
06 - Finish with third bread slice mayo-side down. Gently press sandwich together, secure with toothpicks at each corner, and slice diagonally into halves or quarters.
07 - Assemble second sandwich following same process. Serve immediately while bread remains crispy and bacon warm.