Adding the ingredients in the right order is the secret to this recipe's success; the ones that take longest to cook go in first. The optional layer of seaweed (available from most fishmongers) imparts a salty ocean essence and keeps the potatoes off the bottom of the pot.
Combine onions, garlic, pale ale, and water in a 16-quart stockpot. Cover with a layer of seaweed (or place a steamer basket on top of onions). Add potatoes, chorizo, and 1 tablespoon salt. Bring to a boil. Add lobsters; cook over high heat, covered, for 15 minutes. Add clams and corn; cook, covered, for 6 minutes. Add mussels and shrimp; cook, covered, until clams and mussels open and shrimp are cooked through, 4-8 minutes.
Remove seafood, corn, potatoes and chorizo using tongs, and transfer to large platters or rimmed baking sheets. Discard seaweed and any unopened clams and mussels. Strain liquid through a sieve into a bowl; add butter, swirling to melt. Squeeze lemons over clambake.