Direction

1. Peel and devein shrimp, leaving tails intact. Place shells in a large saucepan; set shrimp aside.
2. Add water, garlic, and peppercorns to the large saucepan with the shells. Zest lemon and set the zest aside. Halve lemon and squeeze juice into the water mixture, then add the lemon halves to the saucepan. Bring to just a boil over high, then reduce the heat to maintain a simmer. Cook, uncovered and undisturbed, to infuse liquid, about 10 minutes. Remove and discard only the shells.
3. Prepare an ice bath and place it next to the stove. Add shrimp to the simmering water and poach on low heat, stirring often, until shrimp are bright pink and just turn opaque, 5 to 6 minutes. Using a slotted spoon, transfer shrimp from poaching liquid using a slotted spoon; immediately place in a large bowl of ice water to stop the cooking process.
4. Stir together shrimp and olive brine in a medium bowl. Cover and chill for 20 minutes.
5. Meanwhile, stir together cocktail sauce, gin, and vermouth in a small bowl. Arrange 3 olives on a cocktail skewer. Divide cocktail sauce into 4 martini glasses, hang 3 to 4 shrimp around the edge of a martini glass. Sprinkle with lemon zest and chopped parsley. Serve immediately.