Add 6 quarts of water to a 12-quart pot, along with bay leaves, salt, peppercorns, and the old bay seasoning. Bring to a boil. Add potatoes and cook for 7-8 minutes. Add sausage, clams, and onions, and boil for five more minutes. Add corn and continue boiling until cooked and the potatoes are tender.
Add the shrimp and simmer until no longer pink. Drain everything.
Transfer the shrimp boil to a large serving dish and dust with more old bay seasoning. Drizzle the seafood boil sauce over the top. Decorate with fresh parsley and lemon slices.