In large bowl combine chicken, celery, red onion, cranberries, walnuts, and grapes. Gently mix until combined.
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Mix Greek yogurt, mayo, mustard, and spices together in medium separate bowl.
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Pour dressing mix over chicken mixture and carefully stir (do not crush grapes) to coat. Refrigerate until cold.
Meanwhile, lightly spray apple slices with natural cooking spray and grill for about 2 minutes.
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Assemble sandwich by spreading about 1 cup of chicken mixture on one slice bread, layer with two slices of grilled apple, and top with another slice of bread.
ENJOY!