Prepare the Chicken Salad Filling
In a large mixing bowl, combine the shredded or diced chicken, mayonnaise, Dijon mustard, and lemon juice. Stir until the chicken is evenly coated.
Add Vegetables and Seasonings
Add the chopped celery, red onion (if using), and fresh parsley to the chicken mixture. Stir again until well combined. Taste and season with salt and pepper as needed.
Assemble the Sandwiches
Lay two slices of bread on a clean surface. Spoon a generous amount of chicken salad onto one slice. Top with lettuce, tomato, and pickles if desired. Place the second slice of bread on top to complete the sandwich.
Serve
Serve immediately, or wrap the sandwiches in parchment paper or plastic wrap for an easy-to-go meal.