01 - In a large stockpot, combine water, beer, Cajun seasoning, Old Bay, smashed garlic, bay leaves, salt, black pepper, and cayenne. Bring to a rolling boil over high heat, stirring to dissolve seasonings.
02 - Add halved potatoes and quartered onion to the boiling liquid. Reduce heat to medium, cover, and simmer for 12–15 minutes until potatoes are fork-tender but not falling apart.
03 - Stir in corn pieces, sliced sausage, and lemon slices. Maintain simmer for 7–8 minutes until corn becomes bright yellow and sausage is heated through.
04 - Add shrimp to the pot and cook for 2–3 minutes only, just until shrimp turn pink and opaque. Remove immediately to prevent overcooking and toughening.
05 - Drain the boil completely using a slotted spoon or spider strainer, discarding bay leaves. Transfer to a large serving platter or spread over newspaper for casual presentation. Drizzle with melted butter, garnish with parsley, and serve with lemon wedges.