1. Prepare the Chicken: If using leftover roast chicken with skin and bones, remove and discard them. Shred or chop the chicken into bite-sized pieces—aim for pieces about ½-inch in size. Place in a large mixing bowl.
2. Prep the Vegetables: Wash and finely dice the celery into small pieces (about ¼-inch). If using red onion, dice finely and soak in cold water for 10 minutes, then drain and pat dry (this mellows the harsh bite). Chop fresh herbs finely.
3. Mix the Dressing: In a small bowl, whisk together mayonnaise, lemon juice, Dijon mustard, and garlic powder until smooth.
4. Combine Everything: Add the celery, onion, and fresh herbs to the bowl with the chicken. Pour the mayo mixture over everything. Using a large spoon or rubber spatula, gently fold everything together until the chicken is evenly coated with dressing and the vegetables are well distributed.
5. Season to Taste: Add salt and black pepper to taste, starting with ¼ teaspoon salt and ⅛ teaspoon pepper, then adjust as needed. Taste and adjust any seasonings—add more lemon juice for brightness, more mayo for creaminess, more herbs for freshness.
6. Chill (Optional but Recommended): Cover and refrigerate for at least 30 minutes to allow flavors to meld. This step is optional but makes a noticeable difference in flavor! The chicken salad can be made up to 3 days ahead.

Assemble the Sandwiches:

7. Prepare the Bread: If desired, lightly toast the bread for added structure and to prevent sogginess. Leave it untoasted for classic soft texture.
8. Build the Sandwich: Place lettuce leaves on one slice of bread (this creates a barrier between the bread and moist chicken salad, preventing sogginess). Pile a generous amount of chicken salad on top of the lettuce—about ¾ cup per sandwich.
9. Add Extras (Optional): Top with tomato slices, avocado, crispy bacon, or any other desired additions.
10. Top and Serve: Place the second bread slice on top. Press down gently. Cut in half diagonally and serve immediately, or wrap tightly for later.