Cook the Chicken: Use leftover cooked chicken or cook fresh chicken breasts. Boil or roast the chicken until fully cooked, then shred or dice it into bite-sized pieces.
Prepare the Vegetables and Fruits: While the chicken is cooling, chop the celery, red onion, and grapes (if using). Set them aside.
Mix the Salad: In a large mixing bowl, combine the shredded chicken, chopped celery, red onion, and grapes. Add the mayonnaise and Dijon mustard, and stir until all the ingredients are well coated. Season with salt and pepper to taste.
Chill and Serve: Cover the bowl and refrigerate the salad for about 30 minutes before serving. This allows the flavors to meld. Garnish with fresh herbs before serving.