Remove the meat from the rotisserie chicken, discarding the skin and bones. Shred or dice it into bite-sized pieces — you want a mix of chunky pieces and smaller shreds for the best texture. Measure out 3 cups into a large mixing bowl.
Finely dice the celery, halve the red grapes, and mince the red onion. Toast the pecans in a dry skillet over medium heat for 2–3 minutes, shaking the pan, until fragrant and lightly golden; let cool, then roughly chop. Chop the fresh dill. Add everything to the bowl with the chicken.
In a small bowl, whisk together the mayonnaise, Greek yogurt, dijon mustard, lemon juice, salt, and black pepper until completely smooth and creamy. Taste and adjust — a little more lemon brightens it, a pinch more salt wakes everything up.
Pour the dressing over the chicken mixture. Using a spatula or large spoon, fold gently until every piece is evenly coated. Be careful not to mash or overmix — you want distinct chunks of chicken, not a paste.
Cover and refrigerate for at least 30 minutes (2 hours is even better) so the flavors meld and the salad firms up. Give it a gentle stir, taste for seasoning, and serve in sandwiches, lettuce wraps, or over a bed of greens, garnished with extra dill.
