Shrimp Cocktail

Fill a large bowl halfway with ice and add water to cover; set aside. Cut one lemon into wedges and set aside. In large pot combine 6 cups water, salt, sugar, onion, and 5 sprigs parsley. Halve and juice remaining lemon and add to pot. Bring to a boil, then turn off heat and add shrimp to pot.  Let sit until shrimp are pink and cooked through, about 3 minutes. Remove shrimp from the pot with a slotted spoon or spider into the bowl of ice. Make the cocktail sauce: Mix all ingredients in a small bowl until fully combined.  Serve shrimp with lemon wedges, cocktail sauce, and garnish with parsley.