In a large mixing bowl, whisk together the mayonnaise, balsamic vinegar, and sugar until smooth and creamy. This creates a tangy-sweet dressing that perfectly balances the salad's fresh flavors.
Add the broccoli florets, halved grapes, and finely diced red onion to the bowl. Toss everything thoroughly to ensure the vegetables and fruit are evenly coated with the dressing.
Gently fold in the chopped pecans, reserving a small handful for garnish. The pecans add a delightful crunch and nutty undertone to the salad.
Refrigerate the salad for at least 30 minutes before serving to allow the flavors to meld and the broccoli to slightly soften. When ready to serve, give the salad a quick stir to redistribute the dressing.
Sprinkle the reserved pecans over the top for a beautiful finishing touch, and enjoy this refreshing, crisp, and slightly sweet salad!