This is one of my favorite side dishes for a BBQ . . . it's creamy, full of flavor, and easy to throw together. In a large bowl combine the macaroni, celery, onion, parsley and tomato, if using. In a small bowl, whisk together the mayonnaise, mustard, sugar, vinegar, sour cream and salt. Pour the dressing over the salad and stir to combine. Season with salt and pepper to taste. Serve. Store covered in the refrigerator, for up to 3 days.