To make a roux, whisk together oil and flour in a large heavy saucepan (cast iron is perfect, but any heavy-bottomed pot or pan will do) over medium to medium-low heat. Once combined, it is easier to use a wooden spoon to stir. Stir continuously for at least 20 minutes, until mixture turns a dark caramel color. It should darken very slowly. If it darkens too fast, turn down the heat. The roux will burn if the heat is too high or you don't stir continuously. Be sure to really stir all the way up to the edges of the pot. You don't want any of the mixture to stay in one spot for too long.
Turn heat to low and add onion, green pepper, and celery to roux. It will start to sizzle loudly and darken some in color. Cook 5 minutes, or until vegetables are soft.
Add garlic and cook 30 seconds.
Add clam juice, tomatoes, Cajun seasoning, Worcestershire sauce, black, white, and cayenne pepper, salt, thyme, and Tabasco. Bring to a boil and simmer 15 minutes.
Add shrimp and cook 2 to 3 minutes. Turn heat off and add butter. Stir to mix it in.
Serve with green onions and white rice and additional hot sauce.
I keep things simple and use a bottle of clam juice, but you could boil your shrimp peels and make a shrimp stock instead. Be sure to use a heavy-bottomed pan or pot for making the roux. Cast iron works perfectly.