Place the 1 ½ lb boneless skinless chicken breasts in a saucepan and cover with cold water by about 1 inch. Add the 1 tsp salt for poaching water, 1 whole bay leaf, and 4 whole black peppercorns. Bring to a boil over medium-high heat, then immediately reduce the heat to a gentle simmer.

Simmer the 1 ½ lb boneless skinless chicken breasts for 12–15 minutes, or until cooked through and no longer pink in the center. The thickest part should reach an internal temperature of 165°F. Remove from the pot and let cool for 10 minutes.

Once cool enough to handle, shred or chop the 1 ½ lb boneless skinless chicken breasts into bite-sized pieces using two forks or a knife. Set aside.

In a large mixing bowl, whisk together the ½ cup mayonnaise, 2 tbsp Dijon mustard, 1 tbsp fresh lemon juice, 1 tsp salt for poaching water, and 4 whole black peppercorns until smooth and well combined.

Add the shredded 1 ½ lb boneless skinless chicken breasts, diced stalks celery finely diced, ¼ red onion finely diced, ¼ cup sweet pickle relish, and 2 tbsp fresh parsley chopped (if using) to the mixing bowl with the dressing. Stir everything together until evenly coated. Taste and adjust seasoning — add more 1 tsp salt for poaching water, 1 tbsp fresh lemon juice, or 2 tbsp Dijon mustard to your preference.

For best flavor, cover the chicken salad and refrigerate for at least 15 minutes before assembling.

Lay a 4 pieces large flour tortillas 10-inch flat on a clean surface. Place a large large romaine lettuce leaves leaf in the center, leaving a 2-inch border at the bottom. Spoon about ¾ cup of chicken salad onto the lettuce. Add 1 tomato thinly sliced slices and ½ avocado sliced if using.

Fold the bottom edge of the pieces large flour tortillas 10-inch up over the filling, then fold in the two sides, and roll firmly away from you to form a tight wrap. Slice in half on a diagonal and serve immediately, or wrap tightly in plastic wrap or aluminum foil to pack for later.