6 ounces seafood herb and spice seasoning, such as Old Bay Seasoning
1 tablespoon dried thyme
1 teaspoon crushed red pepper flakes
2 bay leaves
1 lemon, cut crosswise into 1/4-inch-thick slices
2 large onions, cut crosswise into 1/4-inch-thick slices
5 pounds medium shrimp with heads, rinsed
Fill a 20-quart pot three-quarters full with cold water; add spice mix, thyme, chili pepper flakes, lemon, and onions. Bring water to a rapid boil over high heat.
Using tongs, carefully add shrimp to pot, and reduce heat to a simmer. Cook, stirring occasionally, until shrimp are opaque and shells are pink, about 7 to 10 minutes. Remove shrimp using a slotted spoon or strainer. Serve hot.