Fill a large pot with water and bring it to a boil. Add the Old Bay seasoning, onions, garlic, and lemon halves to the water and let it simmer for about 10 minutes to infuse the flavors.
Add the potatoes to the pot and cook for about 10 minutes, or until they are just starting to soften.
Add the sausage and corn to the pot and continue cooking for another 5 minutes.
Finally, add the shrimp to the pot and cook for about 2-3 minutes, or until they turn pink and are cooked through. Be careful not to overcook the shrimp, as they can become rubbery.
Once everything is cooked, carefully drain the pot and transfer the shrimp, sausage, corn, and potatoes to a large serving dish.
Garnish with fresh parsley and serve with melted butter on the side for dipping, if desired.