Fill a large pot with 5 quarts of water. (Add more if needed; the volume will depend on the size and shape of your pot.) Add the beer, onions, two of the lemons, seafood boil, bay leaves, peppercorns and 2 tablespoons salt. Cover and bring to a rolling boil. Reduce the heat to a lazy simmer and let cook until the flavors marry, about 15 minutes. The liquid should be highly seasoned and aromatic.
Once the cooking broth is flavorful, add the potatoes and cook, covered, until just tender, about 10 minutes. Add the sausage and corn, and cook until tender, 5 to 7 minutes. Add the shrimp and cook until just pink, about 3 minutes. Drain through a large colander.
Transfer the shrimp, sausage, and vegetables to a large platter or tray. Serve immediately with hot sauce and melted butter on the side.