• Step 1: Cook the macaroni according to package instructions. Drain and rinse with cold water to cool completely.
• Step 2: In a large bowl, whisk together the mayonnaise, sour cream, vinegar, Dijon mustard, sugar, salt, and black pepper until well combined.
• Step 3: Add the cooled macaroni, diced hard-boiled eggs, diced celery, and diced red onion to the dressing. Gently toss to coat everything evenly.
• Step 4: Cover and refrigerate the macaroni salad for at least 1 hour to allow the flavors to meld.
• Step 5: Just before serving, stir in the chopped fresh parsley, if using.