Prep the chicken. Shred the cooked chicken using two forks, pulling it into pieces while still warm if possible — warm chicken shreds more cleanly and absorbs the dressing better than cold chicken. If using canned chicken, drain it extremely well and press out all excess liquid before flaking. Allow any hot chicken to cool completely before mixing so it does not melt the dressing.
Wash, dry, and halve the grapes. Rinse the grapes under cool water, then spread them on a clean kitchen towel and pat completely dry. This step is critical — any surface moisture will thin the dressing over time. Once dry, halve each grape lengthwise and set aside.
Prep the remaining ingredients. Dice the celery into small, even ¼-inch pieces. Thinly slice the green onions. Roughly chop the pecans or walnuts to about pea-size pieces. If toasting the nuts, do it now in a dry skillet over medium heat for 3–4 minutes until fragrant, then set aside to cool.
Make the dressing. In a large mixing bowl, whisk together the mayonnaise, Dijon mustard, fresh lemon juice, salt, black pepper, and dried ginger if using. Stir in the fresh parsley or dill. Taste the dressing before adding anything else — it should be creamy, slightly tangy, and well-seasoned. It is much easier to adjust flavor at this stage than after the chicken and grapes are in the bowl.
Fold everything together. Add the shredded chicken, halved grapes, diced celery, and green onions to the bowl with the dressing. Using a large rubber spatula, gently fold everything together with slow, sweeping folds until every piece is evenly coated. Do not stir aggressively — you want the grapes to remain whole and intact. Hold the toasted nuts for now.
Chill the salad. Cover the bowl tightly with plastic wrap and refrigerate for at least 30–60 minutes before serving. This step is important — the chicken absorbs the dressing, the flavors develop, and the texture firms up. Overnight chilling produces an even better result.
Taste, add nuts, and serve. After chilling, taste the salad again and adjust salt, lemon juice, or Dijon as needed — cold temperatures mute flavor and the salad may need a small adjustment. Add the toasted pecans or walnuts right before serving. Pile onto croissants, sandwich bread, or lettuce wraps and enjoy immediately.