Fill a large saucepan two-thirds full with water and add the onion halves. Bring to a boil over medium high heat. 

While waiting for the water to come to a boil, to a large bowl add 1 quart of cool water, the salt, the lemon juice and liquid crab boil. Stir to dissolve salt. Add 2 quarts of ice and set the seasoned ice bath aside.

When the water comes to a boil, add the shrimp and cook just until the water returns to a boil. Immediately remove from the heat and allow shrimp to sit in the hot water until just cooked through, about 1 minute longer. Drain the shrimp promptly and transfer them to the ice bath until completely cool. Once cooled, drain the shrimp in a colander and pat them dry. Refrigerate until ready to serve.

When ready to serve, toss the shrimp with a generous amount of the Remoulade Sauce – it should thoroughly coat the shrimp. (Save any remaining Remoulade Sauce for another purpose.*) Serve the shrimp remoulade in chilled bowls, garnished with a sprinkling of chives if desired.