Toast the nuts. Warm a dry skillet over medium heat.
Add the walnuts and toast for 3–4 minutes, stirring, until fragrant. Cool, then chop. This step adds richer flavor and better crunch.
Make the dressing. In a large bowl, whisk together mayonnaise, Greek yogurt, lemon juice, Dijon, honey, salt, and pepper.
Taste and adjust. You want a creamy, slightly tangy dressing with a gentle sweetness.
Prep the produce. Dice the apples (no need to peel unless you prefer), slice the celery, and halve the grapes. Chop the parsley and slice the green onions if using.
Combine the salad. Add chicken, apples, celery, grapes, walnuts, parsley, and green onions to the bowl with the dressing.
Toss gently until everything is evenly coated.
Adjust and chill. Taste and add more salt, pepper, or lemon as needed. For best flavor, cover and chill for at least 30 minutes to let the flavors meld.
Serve. Spoon into lettuce cups, pile on toasted sourdough, or serve over mixed greens. Add extra walnuts or a squeeze of lemon on top if you like.