1. Combine all sauce ingredients in a medium saucepan.
2. Bring to a boil on medium-high heat.
3. Reduce heat; simmer for 10 minutes or until onion is tender and sauce thickens slightly, stirring frequently. Cool.
4. Fill a large bowl with crushed ice; place bowl of cocktail sauce in centre of ice.
5. Arrange shrimp on ice around bowl of sauce.