Bring a large pot of salted water to a boil over high heat. Add the elbow macaroni and cook according to the package directions until al dente, usually around 7-8 minutes. Drain and rinse under cold running water.

In a medium bowl, whisk together the mayonnaise, yellow mustard, white vinegar, salt, pepper, granulated garlic, and onion powder until smooth. Stir in the diced red onion, celery, and green bell pepper.

In a large mixing bowl, add the cooled elbow macaroni, then pour in about half of the prepared dressing. Gently fold until the pasta is evenly coated.

Cover the bowl with plastic wrap or a lid, and refrigerate for at least 2 hours or ideally overnight.

After chilling, remove and stir in the remaining dressing. Taste and adjust seasoning with salt and pepper.

Transfer to a serving bowl and garnish with additional chopped vegetables or parsley. Serve chilled.