Creating the Chicken Salad Sandwich is straightforward when you follow these simple steps:

1. Prepare the Chicken: If you haven't done so already, cook the chicken by boiling or baking until fully cooked. Then chop or shred it into bite-sized pieces.
2. Make the Dressing: In a large mixing bowl, combine the mayonnaise, Greek yogurt, Dijon mustard, and lemon juice. Mix well until smooth.
3. Add Veggies: Stir in the chopped celery, red onion, and green bell pepper into the dressing. This brings freshness and crunch to the salad.
4. Combine: Add the cooked chicken to the vegetable mixture. Stir gently to combine so that all the chicken is coated with the dressing.
5. Season: Add salt and pepper to taste. Adjust the seasoning based on your preference.
6. Assemble Sandwiches: Lay out your slices of bread. On one slice, add a generous amount of the chicken salad mixture.
7. Add Garnish: Top with a leaf of lettuce and sliced tomatoes if desired.
8. Complete the Sandwich: Place a second slice of bread on top to complete the sandwich. Cut in halves or quarters for serving.
9. Serve Fresh: Enjoy the Chicken Salad Sandwich immediately or refrigerate for later.

These easy steps will lead you to prepare a fantastic Chicken Salad Sandwich that everyone will enjoy!