In a large pot, add water along with the boil bag, oranges, lemons, onion, and a generous sprinkle of Old Bay seasoning.
Bring the pot to a boil, then add the potatoes first and cook for 5–7 minutes, until they just start to soften. Next, add the corn and continue cooking for about 5 minutes. Add the sausage and let it heat through. Finally, add the shrimp and cook just until they turn pink and opaque.
Remove everything from the pot and set it aside. Reserve about ¼ cup of the broth for the sauce.
In a pan, melt butter over medium heat.
Add garlic and lemongrass and sauté until fragrant.
Stir in fish sauce, Cajun seasoning, Old Bay seasoning, and chili flakes.
Pour in the reserved broth and mix until the sauce is rich and slightly emulsified.
Pour the garlic butter sauce over the shrimp boil and toss everything until fully coated and glossy.
Serve hot, family-style, and enjoy immediately.