Clean the fish. Rinse the shrimp and crab legs. The clams should be scrubbed and soaked in salted cold water for 30 minutes , then rinse them until the water clears. Set aside.
Create your broth. In a large stockpot, fill it about halfway (about one gallon) with water. Include the garlic, lemongrass ginger, fish sauce old bay, lemon and salt. Bring to a boil on high temperatures. Then lower the heat to medium-low , and let it simmer for about 15 minutes.
Include the fish and the vegetables. Add the potatoes and bring back to a mild simmer. Cover and cook at medium-high for 15 mins. After 15 minutes adding the sausage and clams. Cover the pot, and simmer at medium-high temperature for 5 minutes. Then, add the shrimp, crab legs and corn. Cover the pot and allow the dish to sit on low heat until shrimp and clams have cooked for about 3-5 mins. It is done when shrimp turn red and the clams break. With a colander, remove the seafood, then discard the water. Remove the lemongrass and lemons, ginger and garlic. Set aside.
Prepare the garlic butter sauce. In a small mixing bowl combine Old bay, the paprika black pepper, cayenne pepper, and sugar until well-combined. In a large pan at medium-low temperature, add the garlic and butter. Cook the garlic until it is fragrant. Add the spice blend, juice of a lemon, and Louisiana hot sauce. Stir until combined. Switch off the heat and put aside.
Mix everything together. Transfer the seafood and the vegetables into an enormous mixing bowl and mix the seafood in the butter sauce until it evenly coating the seafood. Serve with the salt and pepper.