Combine first 3 ingredients in a stainless steel or glass bowl. Cover and refrigerate for 2 hours.
Combine onion and next 5 ingredients in a separate bowl. Drain shrimp, reserving half of liquid. Stir shrimp and reserved liquid into onion mixture. Spoon into individual glasses; garnish, if desired.
Note: For firmer texture, cook shrimp in boiling water to cover for 30 seconds. Plunge into ice water to stop the cooking process; drain and proceed with recipe.