Macaroni Salad

3/4 teaspoon dry mustard
3 tablespoons sour cream
2 cups dry elbow macaroni, cooked, rinsed, and drained
1/4 cup minced red onion, soaked in cold water for 5 minutes, drained
1/3 cup diced celery
1/2 teaspoon kosher salt, plus more to taste
1/2 cup prepared mayonnaise
1/2 cup diced vine-ripened tomato (optional)
1 tablespoon minced flat-leaf parsley
1 1/2 teaspoons sugar
1 1/2 tablespoons cider vinegar
Freshly ground black pepper

Directions

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.