01 - Preheat a skillet over medium heat and cook turkey bacon until browned and crisp, about 3 to 4 minutes per side. Drain excess fat on paper towels.
02 - Lightly butter one side of each bread slice, then toast in a skillet or toaster until golden brown.
03 - Spread mayonnaise evenly on the untoasted side of each toasted bread slice. Add Dijon mustard if desired.
04 - On one slice with mayo side up, layer lettuce, half the turkey slices, tomato, onion, and half the crispy turkey bacon. Place second bread slice (mayo side up) on top, then repeat the same layering. Finish with the third bread slice, mayo side down.
05 - Insert toothpicks to hold sandwiches together and slice into halves or quarters as preferred.