If using a rotisserie chicken, remove the skin and bones, then shred the meat into bite-sized pieces. If starting from raw, poach two boneless, skinless chicken breasts in salted water for 15 minutes or until cooked through, then cool and shred. You want about 3 cups of tender, juicy chicken.
Halve the red grapes—they should be about the size of your chicken pieces. Dice the celery into small, uniform cubes. Finely chop the red onion so it doesn’t overwhelm the salad. Roughly chop the toasted almonds, leaving some small pieces for crunch and a few larger ones for texture.
In a small bowl, whisk together the Greek yogurt, lemon juice, Dijon mustard (if using), salt, and pepper. Stir in the fresh dill. Taste and adjust seasoning—the dressing should be tangy and well-seasoned. Let it sit for a minute so the dill perfume blooms.
In a large mixing bowl, combine the shredded chicken, halved grapes, diced celery, chopped red onion, and toasted almonds. Pour the dressing over the top and gently fold everything together with a rubber spatula until every piece is coated. Be careful not to break the grapes or crush the almonds.
Cover the bowl with plastic wrap or a lid and refrigerate for at least 30 minutes before serving.