Prepare the chicken by shredding or dicing it into bite-sized pieces. If using raw chicken, boil or grill until fully cooked, then let cool before shredding.
Chop the celery and red onion finely. Halve the grapes.
In a large mixing bowl, combine Greek yogurt, mayonnaise, Dijon mustard, and lemon juice. Stir until smooth and creamy. Add salt and pepper to taste.
Combine the prepared chicken, chopped vegetables, and sliced almonds with the dressing. Gently fold until evenly coated. Refrigerate for at least 30 minutes before serving.