In a large mixing bowl, combine halved seedless red grapes, diced cucumber, sliced celery, and finely chopped red onion. Set aside.

In a separate small bowl, whisk together mayonnaise, Dijon mustard, and white wine vinegar until smooth.

Season the dressing with salt and black pepper to taste, adjusting as needed.

Pour the dressing over the grape mixture in the large bowl and gently fold until coated.

Cover the bowl with plastic wrap and refrigerate for at least 30 minutes.

Before serving, sprinkle with toasted slivered almonds or chopped pecans and gently fold in.