If using raw chicken, poach or bake until cooked through, then let cool and shred or dice. Rotisserie chicken works great too.
Toast the pecans in a dry skillet over medium heat for 3-4 minutes until fragrant, then chop.
In a large bowl, whisk together mayonnaise, Greek yogurt, lemon juice, Dijon mustard, salt, and pepper.
Add the chicken, grapes, pecans, celery, and red onion. Stir until everything is well coated.
Taste and adjust seasoning. Cover and refrigerate for at least 30 minutes before serving for best flavor.