If not using leftovers from a boil, follow instructions on boil packet (cut the recipe in half since we are not boiling 15lbs of delicious seafood) Once you added your water and seasoning, toss in the onion, lemons and celery. Add 2 capfuls of crab boil if using and bring to a boil. Taste and adjust seasoning to your liking. If you want a little more heat, add a pinch of cayenne pepper. Prepare a bowl with water and ice, place aside.
Meanwhile, in a bowl combine the mayo, buttermilk, creole mustard, seasoning, lemon juice, salt and mix well. Add the potatoes, corn, green beans, shrimp, onions, celery and green onions. Combine and enjoy.