Bring a large pot of water to a boil. Add the shrimp and return the water to a boil. Remove from the heat, cover and let stand for 5 minutes. Drain and spread the shrimp on a shallow platter. Refrigerate until completely chilled.
When the shrimp are completely cooled, peel and devein. Put the shrimp in a shallow plastic container and arrange the onions evenly over them. Combine the remaining ingredients in a bowl and whisk to blend.
Pour the mixture over the shrimp and onions, cover securely with plastic wrap, and refrigerate for at least 24 hours, stirring two to three times. Drain and serve.
