Pat chicken breasts dry with paper towels. If breasts are uneven, place between plastic wrap and gently pound to an even thickness (about 3/4 inch) so they cook evenly. Season both sides with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper.

Grill or pan-sear

Heat a grill or heavy skillet over medium-high heat. Add 1 tbs olive oil to the skillet (or brush oil on grill grates). Cook chicken 4–6 minutes per side until internal temperature reaches 165°F (74°C) and juices run clear. Remove from heat and rest on a cutting board for 5–10 minutes to retain juices.

Once rested, slice or roughly chop the chicken into 1/2–3/4 inch pieces. Place in a large mixing bowl.

Make dressing and combine salad

Prepare dressing

In a small bowl whisk together 1/2 cup mayonnaise, 1/4 cup plain Greek yogurt, 1 tbs Dijon mustard, 1 tbs fresh lemon juice, 1/4 tsp salt and 1/4 tsp black pepper until smooth and creamy. Taste and adjust seasoning as needed.

Chop vegetables

Finely dice 1 stalk celery and measure 2 tablespoons finely diced red onion (reserve additional onion slices for assembly). Chop 2 tsp fresh parsley.

Combine

Add the diced celery, diced red onion, 1/2 cup dried cranberries, 1/2 cup toasted chopped walnuts, and chopped parsley to the bowl with the chopped chicken. Pour the dressing over the mixture and gently fold until everything is evenly coated. Taste and adjust salt/pepper or lemon if desired. Chill in the refrigerator 15–30 minutes for flavors to meld (optional).

Toast bread and assemble sandwiches

Prepare bread

Heat a skillet or griddle to medium. Lightly butter or brush 1 tbs total butter/olive oil across the slices of bread on one side. Toast bread, buttered side down, 2–3 minutes until golden and crisp. Alternatively, use a toaster then spread butter if preferred.

Assemble

Lay out 4 slices of toasted bread. On two slices place 2 leaves of lettuce, 2–3 slices of tomato each, and 2 slices of red onion each. Portion the chicken salad evenly (about half the mixture per sandwich) on top of the vegetables. Top with remaining bread slices (toasted side out).

Cut sandwiches in half if desired and serve immediately. Leftover chicken salad keeps covered in the refrigerator for 2–3 days.