For stock, simmer shrimp shells in about 6½ cups of water for 20 minutes.
In a 6-8quart pot, make roux: Heat oil, then sprinkle in flour gradually, as you whisk. Cook flour and oil on medium heat about 20 minutes, to a very dark reddish brown. Stir constantly, especially towards the end. (Be gutsy. The best roux is quite dark, but don’t let it burn. Dark roux is what makes this gumbo great. )
Add vegetables to the roux. Cook 10 minutes (or 15 minutes with okra). Add cooled stock slowly.
Add herbs, spices and clam juice. Simmer 15 minutes, then another 30-40 more with Andouille sausage. Add the seafood at the end, to avoid overcooking.
Makes about 10 servings. Top with chopped green onions. Many like to serve over rice. Goes great with a baguette or cornbread.