FOR THE SHRIMP

2 lb. Ice
6 c. water
2 tbsp. kosher salt
2 tbsp. granulated sugar
1/2 small onion, peeled and halved
1 tbsp. small bunch parsley, divided
2 lemons, divided
2 lb. shrimp, peeled and deveined

FOR THE COCKTAIL SAUCE

1/2 c. mild chili sauce, such as Heinz
1/2 c. ketchup
1 tsp. lemon zest
2 tbsp. fresh lemon juice
1 tbsp. horseradish
1 tsp. Worcestershire sauce
2 dashes hot sauce, such as Tabasco
1/4 tsp. celery salt