01 - In a medium saucepan, combine water, lemon slices, salt, and black peppercorns. Bring to a rolling boil over high heat.
02 - Add shrimp to boiling liquid and reduce heat to maintain a gentle simmer. Cook until shrimp are pink and just opaque throughout, approximately 2 to 3 minutes.
03 - Using a slotted spoon, transfer cooked shrimp immediately to a bowl filled with ice water. Allow to cool completely, then drain and pat dry with paper towels.
04 - In a small bowl, whisk together ketchup, prepared horseradish, fresh lemon juice, Worcestershire sauce, hot sauce if desired, and a pinch of salt until fully combined.
05 - Arrange chilled shrimp on a serving platter or divide between two stemmed cocktail glasses. Place cocktail sauce in a small serving dish. Garnish with lemon wedges and fresh parsley. Serve immediately.