Make the seafood stock: In a pot, combine shrimp shells, 6 cups water, halved onion, celery stalk, bay leaf, and peppercorns.
Simmer 25–30 minutes. Strain and keep warm. You should have about 5 cups.
Brown the sausage (optional): In a large heavy pot, brown sliced andouille over medium heat until rendered and lightly crisp.
Remove and set aside, leaving a thin film of fat.
Cook the roux: Add oil to the pot (or supplement with sausage fat) and whisk in flour over medium to medium-low heat. Stir constantly 20–30 minutes until the roux turns the color of dark chocolate. Do not walk away. If it smells burnt, start over.
Add the trinity: Stir in diced onion, bell pepper, and celery. Cook 5–7 minutes until softened and glossy.
Add garlic and cook 1 minute.
Season the base: Stir in salt, black pepper, paprika, cayenne, thyme, oregano, and bay leaves. Let spices bloom 30 seconds.
Build the soup: Slowly whisk in warm seafood stock, a ladle at a time, to avoid lumps. Add tomatoes if using, Worcestershire, and hot sauce.
Return sausage to the pot if using.
Simmer: Bring to a gentle simmer. Add okra if you want a thicker, silkier texture. Cook 25–35 minutes, stirring occasionally, until flavors meld and the gumbo slightly thickens.
Add seafood: Lower the heat to just under a simmer.
Gently add fish pieces and cook 3–4 minutes. Add shrimp and cook another 2–3 minutes, until just pink. Fold in crab last and warm through 1–2 minutes. Do not boil.
Finish: Remove bay leaves.
Stir in parsley and green onions. Taste and adjust salt, pepper, and heat.
Serve: Spoon over hot white rice. Offer filé powder at the table to thicken slightly and add earthiness.
Finish with a squeeze of lemon and extra hot sauce if you like.