1. Peel shrimp; devein, if desired.
2. Cook shrimp in boiling water to cover 3 to 5 minutes or just until shrimp turn pink; drain. Rinse with cold water.
3. Layer shrimp and onions in a large bowl. Whisk together oil and next 7 ingredients; pour over shrimp and onions. Cover and chill 24 hours, stirring occasionally. Stir in parsley just before serving.