Shrimp Cocktail with Dipping Sauces

Name: Shrimp Cocktail with Dipping Sauces
Ingredients: 1/2 cup fresh flat-leaf parsley, leaves and stems separated; 1 tablespoon black peppercorns; 4 cloves garlic, smashed; 1 lemon, halved; 1/2 shallot, halved; Kosher salt; 2 pounds peeled and deveined colossal shrimp, tail on; 1/2 cup mayonnaise; 1 tablespoon drained green peppercorns in brine plus 1 teaspoon brine; 1/2 teaspoon granulated sugar; 1/2 cup ketchup; 1 tablespoon prepared horseradish; 1/4 teaspoon celery seed; 2 dashes Worcestershire sauce