Make dukkah: In a large, heavy frying pan, toast hazelnuts over medium-low heat, stirring often, until browned and fragrant, 10 to 15 minutes. Pour into a bowl and set aside. Toast sesame seeds in pan, stirring often, until golden brown, about 5 minutes. Pour into another bowl and set aside. Toast cumin, coriander, and fennel seeds in pan until fragrant and golden, 2 to 3 minutes. Add to sesame seeds and let cool completely.
Pulse hazelnuts a few times in a food processor to chop coarsely. Add sesame mixture and salt and pulse until coriander seeds are broken up and dukkah is crumbly.
Make salad dressing: In a large bowl, combine yogurt, mustard, lemon juice, salt, and 1/2 cup dukkah.
Add chicken, grapes, onion, parsley, and celery to dressing. Fold together gently but thoroughly with a rubber scraper. Serve on butter-lettuce leaves.
Make ahead: Dukkah, up to 1 month, chilled airtight.