In a large bowl, whisk together the vinegars, black pepper, mustard, pepper flakes and salt. Slowly pour the oil in while whisking. With a spoon stir in parsley and green onions and set aside.
In a large pot of water add the crab boil if using and bring to a boil for 3-4 min. Add the shrimp and cook 2-3 min. until pink. Drain shrimp, shaking off excess water and immediately toss into the mustard sauce. Cover and refrigerate overnight.
Serve with wooden toothpicks or cocktail forks.
