Poach, grill, or bake the chicken breasts. Poach in simmering water 15–20 minutes until no longer pink. Grill 6–8 minutes per side over medium-high heat, or bake at 375°F (190°C) for 25–30 minutes. Let cool, then shred or dice.
Preparing Mix-ins
Finely chop celery and red onion. Halve the grapes if using. Lightly toast almonds in a dry skillet over medium heat for 2–3 minutes until fragrant; cool.
Making the Dressing
In a bowl, combine mayonnaise, Greek yogurt, Dijon mustard, lemon juice, salt, and black pepper. Whisk until smooth. Taste and adjust seasoning.
Combining Ingredients
Add the chicken, celery, red onion, grapes, and almonds to a large bowl. Pour the dressing over and fold gently until evenly coated.
Chilling and Serving
Refrigerate at least 30 minutes to let flavors meld, or serve right away if you're hungry. Garnish with parsley before plating.
Notes
For a lighter version, swap half the mayo for extra Greek yogurt. To make it paleo, replace mayo and yogurt with mashed avocado and a little olive oil. If you want a warm take, omit grapes and serve on rice or roasted vegetables.