Step 1: Poach the Chicken: If starting with raw chicken, place chicken breasts in a saucepan and cover with water. Bring to a simmer over medium heat, then reduce heat to low and poach for 15-20 minutes, or until cooked through. Let cool slightly, then dice into -inch pieces.
Step 2: Prepare the Vegetables and Nuts: While the chicken cools, finely dice the celery. Halve or quarter the red grapes, depending on their size. Coarsely chop the walnuts.
Step 3: Make the Dressing: In a medium bowl, whisk together the mayonnaise, lemon juice, Dijon mustard, kosher salt, and black pepper until smooth and well combined.
Step 4: Combine the Ingredients: In a large bowl, gently combine the diced chicken, diced celery, halved or quartered grapes, and chopped walnuts.
Step 5: Add the Dressing: Pour the dressing over the chicken mixture and gently fold to coat all the ingredients evenly. Be careful not to overmix, as this can make the salad mushy.
Step 6: Chill and Serve: Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld. Before serving, taste and adjust seasoning as needed. Serve on lettuce leaves or croissants, if desired.
For a creamier salad, use full-fat mayonnaise. For a tangier flavor, add an extra teaspoon of lemon juice. Pecans or almonds can be substituted for walnuts. For a different flavor profile, add 1/4 cup of chopped fresh herbs like tarragon or parsley. If using leftover rotisserie chicken, ensure it is not overly seasoned, as this can throw off the flavor balance.