1. Bring 10 qt. water to a boil in a 12-qt. stockpot over medium-high heat; squeeze lemon halves over water, and add squeezed halves to water in stockpot. Add shrimp, and cook 3 minutes or just until shrimp turn pink. Drain and rinse with cold water to stop the cooking process.
2. Peel shrimp; devein, if desired.
3. Layer shrimp, onions, and artichoke hearts in 2 (13- x 9-inch) baking dishes.
4. Stir together olive oil and next 5 ingredients; pour evenly over shrimp mixture. Cover and chill at least 12 hours or up to 48 hours, stirring occasionally. Garnish, if desired, and serve with a slotted spoon.