Fill a large stockpot with 6 quarts water. Add lemons, bay leaves, kosher salt, peppercorns, and Old Bay, cover, and bring to a boil.
Add potatoes, return to a boil, and cook 7 minutes.
Add sausage & onions, return to a boil, and cook 5 minutes.
Add corn, return to a boil, and cook until corn is cooked and potatoes are tender when pierced with a knife, about 5 minutes more.
Add shrimp, remove from heat. Allow all to sit for 4 minutes before straining into a colander.
Remove insert or drain through a very large colander.
Serve shrimp boil on a newspaper-lined table or large platters. Dust with more crab and shrimp boil seasoning, if desired. Serve with Classic Cocktail Sauce and lemon-butter sauce (if using) alongside, plus lemon wedges for squeezing.
