Season the chicken breasts with salt and pepper on both sides.

1. Heat a tablespoon of olive oil in a skillet over medium heat. Once the oil is hot, cook the chicken breasts for 5-7 minutes per side, or until the chicken reaches an internal temperature of 165°F (75°C) and is fully cooked through.
2. While the chicken is cooking, toast the bread slices until golden brown.
3. In a small bowl, mix the mayonnaise with Dijon mustard, if using, to create a creamy spread.
4. Once the chicken is done, remove it from the skillet and let it rest for a few minutes before slicing it into thinner strips.
5. To assemble the sandwich, spread mayonnaise on one side of each slice of toasted bread.
6. On the bottom slice of bread, layer a leaf of lettuce, followed by the sliced chicken, a slice of tomato, and two slices of crispy bacon.
7. Top with the second slice of bread, then repeat the layering process with the remaining ingredients.
8. Finish with the final slice of bread and press down gently.
9. Cut the sandwich in half or quarters, securing each with a toothpick if desired. Serve immediately and enjoy!