MAKE THE BRINE: Fill a large stockpot or dutch oven with ⅔ full with water. Add lemons, Old Bay seasoning, salt, garlic, and onions to the water and bring to a low boil.
COOK THE VEGGIES AND SAUSAGE: Add the potatoes to mixture and boil until they begin to soften (not fully cooked), about 10 minutes. Add the corn and sausage and cook for an additional 3 minutes.
COOK THE SRHIMP: Add the shrimp to the pot and cook for 2 minutes or until the shrimp just become pink and opaque (not longer). Use a ladle to reserve a cup of the brine and then strain the shrimp mixture using a colander. Discard the garlic pieces, lemons and any lemon seeds and remaining brine.
SEASON THE MIXTURE: Return the stock pot to the stovetop and adjust to heat to low. Melt the butter and whisk it into the reserved brine. Add the shrimp back to the pan along with the parsley, toss to coat the whole mixture evenly.
SERVE: Serve immediately and garnish with additional parsley and old bay seasoning.