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.