Bring a large pot of water to a boil, then stir in the 1 tbsp kosher salt. Add the elbow macaroni and cook according to package directions, usually about 8 to 10 minutes. Drain and rinse under cold water until cool, then drain again.

Place the minced red onion in a small bowl of cold water for 5 minutes. Drain well to soften its sharp flavor.

In a small bowl, whisk the mayonnaise, dry mustard powder, sugar, cider vinegar, sour cream, salt, and black pepper until smooth.

In a large bowl, toss the cooled macaroni with the celery, onion, carrots, parsley, and optional tomato. Pour the dressing over and stir until everything is well coated.

Cover the salad and refrigerate for at least 1 hour to allow flavors to meld.

Before serving, stir well, taste, and adjust with extra salt or vinegar as needed. If it looks dry, add extra mayonnaise.