Fill a 12-quart stockpot with 5 quarts of water and 12 ounces of beer.
Stir in the Old Bay seasoning, salt, onion, garlic halves, and the juice and rinds of the halved lemons. Bring to a rolling boil over high heat.
Reduce the heat to medium-high and simmer the broth for 10 minutes to allow the aromatics and spices to infuse the liquid.
Add the red potatoes to the pot and cook until they are just starting to become tender, approximately 12 minutes.
Add the sliced andouille sausage and corn pieces to the boiling liquid and cook for an additional 5 minutes.
Gently stir in the shrimp and cook until they turn pink and opaque, which typically takes 2 to 3 minutes; do not overcook or they will become rubbery.
Immediately drain the mixture through a large colander, discarding the boiling liquid and the spent aromatics (onion, garlic, and lemon rinds).
Spread the boil ingredients onto a large serving platter or a newspaper-lined table, drizzle with melted butter, and garnish with chopped parsley and extra Old Bay seasoning.