Toast pecans in a dry skillet over medium heat for 3–4 minutes; let cool.
Whisk mayonnaise, yogurt, lemon juice, Dijon, salt, and pepper in a large bowl.
Add chicken, celery, and green onions to the bowl; toss to coat.
Fold in grapes, cooled pecans, and dill. Taste and adjust seasoning.
Cover and chill 20 minutes, then serve on bread, croissants, or lettuce.