American Macaroni Salad

2 cups dry elbow macaroni, cooked, rinsed, and drained

1/3 cup diced celery

1/4 cup minced red onion, soaked in cold water for 5 minutes, drained

1 tablespoon minced flat-leaf parsley

1/2 cup diced vine-ripened tomato (optional)

1/2 cup prepared mayonnaise

3/4 teaspoon dry mustard

1 1/2 teaspoons sugar

1 1/2 tablespoons cider vinegar

3 tablespoons sour cream

1/2 teaspoon kosher salt, plus more to taste

Freshly ground black pepper