Per instructions on pasta box, bring 4 quarts of water to a boil. Add 1 tablespoon of salt to water and add pasta. Cook 8-9 minutes for al dente tenderness. Drain pasta and rinse with cool water.
To a large bowl, add pasta, red onion and celery. Set aside.
In a small bowl or lidded jar, add mayonnaise, Dijon mustard, vinegar, garlic powder, salt and pepper. Mix together until creamy. Pour dressing over pasta and veggies. Mix gently making sure all pasta is coated with dressing. Sprinkle on parsley and taste, then adjust seasoning.
Serve immediately. Can be stored, cover in refrigerator for up to two days.