Martha demonstrates how to do a shrimp boil, then answers questions from the audience.
Fill a 20-quart pot 3/4 full with cold water; add Old Bay, thyme, red pepper flakes, lemon, and onions. Bring water to a rapid boil over high heat.
Carefully add shrimp to pot, and return to a boil. Reduce to a simmer, and cook, stirring occasionally, until shrimp are opaque and tails begin to curl, 7 to 10 minutes. Remove shrimp using a slotted spoon or strainer. Serve immediately.