If you do not have pre-cooked chicken: poach two medium boneless skinless chicken breasts (about 1 to 1 1/4 lb raw) in simmering water or broth for 12–15 minutes until cooked through, then let rest 5 minutes and shred or chop into bite-size pieces. Alternatively, use rotisserie chicken for convenience.

In a large mixing bowl combine mayonnaise, Greek yogurt, Dijon mustard, and lemon juice. Whisk until smooth.

Add the shredded chicken, diced celery, finely chopped red onion, chopped parsley, and optional pickles or fruit (if using) to the dressing. Fold gently to coat all ingredients evenly.

Season the mixture with salt and black pepper. Taste and adjust seasoning — add more lemon juice for brightness, more mayo for creaminess, or a pinch more salt/pepper as needed.

Cover the bowl and refrigerate for at least 15–30 minutes to let flavors meld. Can be made up to 24 hours ahead; if refrigerating longer, stir once before assembling and adjust seasoning if needed.

Sandwich Assembly

If toasting bread: lightly butter one side of each slice or brush with olive oil. Heat a skillet over medium heat and toast the bread buttered-side down 1–2 minutes per side until golden, or use a toaster/oven. Alternatively, serve bread untoasted.

Place a lettuce leaf on four slices of bread (the lettuce creates a moisture barrier). Divide the chilled chicken salad evenly among the four sandwiches — about 3/4 cup per sandwich (use all prepared chicken salad).

Top the chicken salad with tomato slices and optional cheese slices if using. Close sandwiches with remaining bread slices, buttered/toasted side out if applicable.

Cut each sandwich in half diagonally or down the middle and serve immediately. If packing for later, wrap tightly in plastic wrap or parchment and refrigerate up to 24 hours.