In a saucepan, poach the chicken by covering it with water and bringing it to a gentle boil. Lower the heat and simmer for about 15-20 minutes, or until the chicken is thoroughly cooked. Remove from water and let it cool, then shred it into pieces.
In a large mixing bowl, combine the shredded chicken, diced celery, and chopped red onion.
In a separate bowl, whisk together the mayonnaise, Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and black pepper. Adjust seasoning to taste.
Pour the dressing over the chicken mixture and gently stir until everything is evenly coated. If desired, fold in the halved grapes and chopped walnuts for added flavor and crunch.
Prepare the bread slices by toasting them lightly on a grill or skillet until golden brown.
Assemble the sandwiches by placing a generous portion of the chicken salad on one slice of bread, topping it with leafy greens, then placing another slice of bread on top.
Repeat with the remaining ingredients to create a total of 4 sandwiches.