Pour the beer into a 44-quart outdoor boil pot (see Cook's Note) and add 16 quarts cold water. Squeeze in the juice from the lemon halves, then toss them in. Add the seafood seasoning and garlic. Tie the thyme sprigs together with kitchen twine and add to the pot. Cover and bring to a boil.
Add the potatoes and cook until they are just tender, about 10 minutes. Add the corn and andouille sausage and simmer until heated through, about 10 minutes. Add the shrimp and cook until pink, about 5 minutes.
Carefully drain the low country boil and transfer to a large serving platter, rimmed sheet pan or straight on the table covered in paper. Drizzle with melted butter, then top with the parsley. Serve with lemon wedges, melted butter and hot sauce.
You can also make this shrimp boil in a very large pot (25 to 44 quarts) on a stovetop.
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