Prep the chicken. If using rotisserie, remove skin and bones, then chop or shred.
For poached chicken, simmer salted water, add breasts, and cook gently until just done, then cool and shred.
Make the dressing. In a large bowl, whisk together mayonnaise, Greek yogurt, Dijon, lemon juice, lemon zest, and a pinch of garlic powder. Season with salt and pepper.
Add the mix-ins. Fold in the celery, red onion, and most of the dill. Stir to combine.
Combine with chicken. Add the chicken and toss until evenly coated.
Taste and adjust salt, pepper, lemon, and dill.
Add crunch. Fold in the sliced almonds or pepitas right before serving for texture.
Chill briefly. Cover and refrigerate for 20–30 minutes so the flavors meld. Sprinkle with extra dill before serving.
Serve. Spoon onto toasted sourdough, stuff into a lettuce wrap, roll in a tortilla, or serve over mixed greens with cucumber and tomatoes.