Dice the cooked turkey breast into uniform bite-size pieces, about ½-inch cubes.
Slice the celery into thin half-moon strips.
Halve the red seedless grapes and set aside.
Roughly chop the toasted walnuts, leaving some larger pieces for textural contrast.
In a large mixing bowl, whisk together mayonnaise, Dijon mustard, lemon juice, salt, and pepper until smooth and well combined.
Add the diced turkey to the bowl with the dressing and gently fold to coat the meat evenly.
Carefully fold in the celery, grapes, walnuts, and any optional ingredients until all components are evenly distributed.
Cover the turkey salad and chill in the refrigerator for at least 20 minutes before serving to allow flavors to meld.
Before serving, taste and adjust seasonings if necessary.