If you don't have cooked chicken, poach or roast boneless chicken breasts or thighs: place 1 lb (about 2 medium breasts) in a pot, cover with water or low-sodium chicken broth, bring to a simmer, and cook 12–15 minutes until cooked through. Let rest 5 minutes, then shred or dice into bite-sized pieces. Cool to room temperature.

In a large mixing bowl combine 1/2 cup mayonnaise and 2 tablespoons Greek yogurt. Whisk until smooth to create a creamy dressing base.

Add 1 tablespoon lemon juice and 1 teaspoon Dijon mustard to the dressing. Whisk to combine so the dressing is tangy and well-emulsified.

Fold in the shredded chicken (3 cups), 1/2 cup finely diced celery, 2 tablespoons finely diced red onion, and 2 tablespoons chopped dill pickle or relish. Stir gently to coat the chicken pieces evenly with the dressing.

Add 2 tablespoons chopped fresh parsley, 3/4 teaspoon salt, 1/2 teaspoon freshly ground black pepper, and 1/4 teaspoon celery seed if using. Mix gently and taste; adjust seasoning by adding more salt, pepper, or lemon juice as needed.

Cover the bowl and refrigerate the chicken salad for at least 30 minutes to let flavors meld. If serving immediately, 10–15 minutes resting still improves flavor. The salad will keep refrigerated in an airtight container for 3–4 days.

Sandwich & Assembly

If desired, lightly toast 8 slices of bread. Spread about 1/4 teaspoon butter or mayonnaise on one side of each slice to prevent sogginess and add flavor.

Place lettuce leaves on 4 of the bread slices (on the side with spread). If using tomato, place slices on top of the lettuce and lightly season the tomato with a pinch of salt (1/8 tsp) and a faint grind of pepper (about 1/16–1/8 tsp).

Spoon about 3/4 to 1 cup of the chilled chicken salad onto each prepared bread slice (over the lettuce/tomato). Spread it evenly to the edges without overfilling so sandwiches are easy to close.

Top with the remaining bread slices, spread-side down. Press gently, then cut each sandwich in half diagonally or straight down the middle, as preferred.

Serve immediately, or wrap sandwiches tightly and refrigerate for up to 24 hours for packed lunches. If making ahead, keep tomatoes separate until just before serving to avoid sogginess.