In a large heavy saucepan, make roux by whisking together oil and flour over medium to medium-low heat. Once combined, stir continuously with a wooden spoon for at least 20 minutes, until mixture turns a dark caramel color. It should darken very slowly. It will burn if the heat is too high or you don’t continually stir it.
Once roux is a deep rich brown, turn heat to low and add onion, green pepper, celery and white part of green onions. Cook 5 minutes until vegetables are soft.
Add garlic and cook 30 seconds.
Add clam juice, tomatoes, Cajun seasoning, Worcestershire sauce, black and cayenne pepper, salt, thyme, bay leaf and Tabasco. Bring to a boil and simmer 15 minutes.
Add shrimp and cook 2 to 3 minutes. Remove bay leaf. Turn off heat and add butter, stirring to mix.
Serve over white rice sprinkled with remaining green onion tops and extra hot sauce.