Prepare the Cocktail Sauce: In a small bowl, combine ketchup, horseradish, Tabasco, gin, celery salt, and lemon juice. Mix well, then refrigerate until ready to serve. Stir in minced parsley just before serving. (Tip: The cocktail sauce can be made up to a day in advance and stored in the refrigerator to save time on the day you plan to serve.)
Create the Poaching Liquid: In a large pot, combine cold water, 1 tablespoon of kosher salt, white wine, peppercorns, coriander seeds, bay leaves, sliced onion, and halved lemon. Bring to a rolling boil over high heat, then reduce heat and let simmer uncovered for 20 minutes to allow flavors to develop.
Strain and Prepare for Poaching: Remove the pot from heat and let the poaching liquid sit for an additional 30 minutes to intensify flavors. Strain the liquid through a fine sieve into a fresh pot, discarding the solids.
Poach the Shrimp: Bring the strained poaching liquid to a rapid boil, then remove from heat. Add the shrimp to the pot, cover, and let them poach for about 3 minutes until tender and opaque.
Chill the Shrimp: Fill a large bowl with cold water, ice, and the remaining kosher salt. Using a slotted spoon, transfer the poached shrimp to the ice bath. Let them chill in the ice bath until cold and opaque, about 3 minutes.
Serve: Arrange the chilled shrimp on a platter with lemon wedges and serve with the chilled cocktail sauce on the side. (For events, you can chill the poached shrimp in the refrigerator after the ice bath and serve when ready.)