Bring 4 quarts of water to boil in a large pot. Add 1 tablespoon of salt and the pasta. Cook until al dente, about 6 to 8 minutes.
Drain pasta in a colander and rinse with cold water until no longer hot. Drain briefly again, leaving a little water on the pasta and transfer to a large bowl. The pasta will absorb this water instead of the mayonnaise which will help it stay creamy.
Add onion, celery, parsley, lemon juice, mustard, garlic powder, and cayenne to bowl with pasta and stir to combine. Let sit 2 minutes to allow the flavors to absorb into the pasta.
Add mayonnaise, stir to combine, and let sit 5 to 10 minutes. Season to taste with salt and pepper.
Serve immediately or refrigerate up to 2 days.