If you haven't cooked the chicken yet, poach or roast 2 medium-sized chicken breasts until cooked through (internal temperature should reach 165°F / 75°C). Allow them to cool, and then shred or dice the chicken.
In a large mixing bowl, combine the cooked chicken, diced celery, and halved grapes.
In a separate small bowl, whisk together the Greek yogurt (or mayonnaise), Dijon mustard, lemon juice, salt, and pepper until smooth.
Pour the dressing over the chicken mixture and stir until all ingredients are well coated.
If using, fold in the chopped almonds or walnuts for added texture.
Taste and adjust seasoning if necessary.
Serve on a bed of lettuce or in a sandwich, if desired.