If not using pre-cooked chicken, poach 2 boneless skinless chicken breasts: place in a saucepan, cover with water or low-sodium chicken broth, add a pinch of salt, bring to a gentle simmer, and cook 12–15 minutes until cooked through. Let rest 5 minutes, then shred or dice into bite-sized pieces. Measure 3 cups.

In a large mixing bowl combine the mayonnaise, Greek yogurt (if using), Dijon mustard, and fresh lemon juice. Whisk until smooth.

Add the celery seed, kosher salt, black pepper, and granulated sugar (if using). Taste and adjust seasoning: more salt, pepper, or lemon to balance.

Fold in the shredded chicken, diced celery, red onion, and sweet pickle relish. Mix gently until ingredients are evenly coated. Chill in the refrigerator at least 30 minutes to let flavors meld (can be made up to 24 hours ahead).

Before assembling, taste again and adjust seasoning (add a little more mayo if you prefer creamier, or a splash more lemon for brightness).

For Assembling the Sandwiches

Lightly butter one side of each bread slice. If you like, toast the buttered side in a skillet over medium heat to lightly brown (about 1–2 minutes per side) for an old-fashioned touch. Alternatively, leave untoasted.

Lay out 4 bread slices, buttered side down. Divide the chilled chicken salad evenly onto the 4 slices (about 3/4 cup per sandwich).

Top the chicken salad with a lettuce leaf, 2–3 tomato slices (if using), and 2 pickle slices (if using). Cover with remaining bread slices, buttered side up.

Press gently, cut each sandwich in half diagonally, and serve immediately. For packed lunches, wrap tightly and keep chilled until serving.