Rub the chicken with the Green Stuff. Grill over medium-high heat for 6-8 minutes per side, until cooked through. Chop the chicken into small pieces and put into a bowl. Refrigerate the chicken while you work on the other ingredients.
Finely chop the celery and onion. Sprinkle them, along with the optional walnuts, into the bowl with the chicken, which should be lukewarm by now. Add the mayonnaise and zest the lemon directly into the bowl. Grind white pepper into the bowl to taste. Stir and serve in a wrap, on low-carb bread, or over salad greens.
Macros per serving, without the walnuts: 264 calories, 1 gram net carbs, 25 grams protein, 19 grams fat.