1. 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 to 8 minutes.

2. 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.