Bring first 5 ingredients to a boil in a large Dutch oven over high heat. Cover and boil 3 to 4 minutes. Add shrimp; remove from heat. Cover and let stand 10 minutes. (Shrimp will turn pink, and shells will loosen slightly.) Drain, discarding lemon, onion, and boil bags.
Note: For testing purposes only, we used Zatarain's Shrimp & Crab Boil.