Step 1: Prepare the Chicken

1. In a bowl, mix olive oil, salt, black pepper, garlic powder, smoked paprika, onion powder, and lemon juice.
2. Coat the chicken breasts evenly with the seasoning mixture.
3. Let the chicken marinate for at least 15-20 minutes (or up to 2 hours for deeper flavor).

Step 2: Grill the Chicken

1. Heat a grill or grill pan over medium-high heat.
2. Cook the chicken for 5-7 minutes per side until golden brown and fully cooked (internal temperature should reach 165°F or 75°C).
3. Remove the chicken from the grill and let it rest for 5 minutes before slicing.

Step 3: Prepare the Bread and Bacon

1. While the chicken is resting, toast the bread slices until golden and crispy.
2. In a skillet, cook the bacon over medium heat until crispy (about 4-5 minutes per side).
3. Place the cooked bacon on a paper towel to absorb excess grease.

Step 4: Assemble the Sandwich

1. Spread mayonnaise (and Dijon mustard, if using) on one side of each toasted bread slice.
2. Layer a piece of lettuce on the first slice of bread, followed by a slice of grilled chicken, a slice of cheese, a tomato slice, and some onion.
3. Add a second slice of toasted bread and repeat the layering process.
4. Top with the final slice of toasted bread.

Step 5: Serve and Enjoy

1. Secure the sandwich with toothpicks if needed.
2. Cut the sandwich into halves or quarters for easy serving.
3. Serve immediately with fries, coleslaw, or a fresh salad.