Stove-Top Clambake

Ingredients

• 2 large or 3 medium onions, cut into large wedges
• 6 garlic cloves
• 1 bottle pale ale or medium-bodied beer
• 1 cup water
• fresh seaweed, well rinsed, for layering (optional)
• 1 1/2 pounds small new potatoes (white, red, or a combination)
• 1 pound hot dried chorizo, cut into 1/2-inch pieces
• coarse salt
• 3 lobsters (1 1/2 pounds each)
• 36 littleneck clams, scrubbed well
• 4 ears of corn, husked and halved
• 2 pounds mussels, debearded and scrubbed well
• 1 1/2 pounds large shrimp (about 30), shell-on
• 2 tablespoons unsalted butter (optional)
• 2 lemons, halved