In a medium mixing bowl, combine the plain Greek yogurt, lemon juice, garlic powder, salt, and pepper. Stir these together until the dressing is smooth and well-combined. This tangy base will act as the healthy binder for your chicken salad.
Add the shredded chicken, dried cranberries, chopped celery, minced red onion, and chopped walnuts into the bowl with the dressing. Use a large spoon or spatula to fold the ingredients together until the chicken is thoroughly coated and the additions are evenly distributed.
Take your slices of whole-grain bread and lightly toast them until they are golden brown. Toasting the bread helps it hold up against the moisture of the chicken salad and adds a nice structural crunch to your sandwich.
Place a layer of fresh spinach or lettuce on two of the bread slices. Scoop a generous portion of the chicken cranberry mixture onto the greens. Place the remaining bread slices on top to close the sandwiches.
Cut the sandwiches in half diagonally using a serrated knife. This makes them easier to eat and presents the colorful filling beautifully. Serve immediately or pack them away for later.