Add the oil to a large heavy-bottomed pot or Dutch oven over medium-low heat. Whisk in the flour to avoid any lumps.
Use a wooden spoon and stir continuously for at least 15 minutes, until mixture turns a dark caramel color. Take your time to avoid burning it.
Turn heat to low and add the onion, green pepper, and celery. Cook vegetables in the roux for 5 minutes, or until vegetables are soft. Add garlic and cook another minute.
Add the clam juice, Worcestershire sauce, Tabasco, cajun seasoning, salt, black and cayenne pepper and thyme. Bring to a boil, turn heat to low and simmer 15 minutes. If you prefer it thinner, add 1/4 cup seafood or chicken stock (optional).
While the étouffée simmers, heat a saute pan hot over medium high heat. Add a little oil to the pan and add the shrimp and cook 1 to 2 minutes per side, or until no longer translucent. Turn heat off and add butter. Swirl to coat shrimp. Transfer shrimp into etouffee and toss to mix.
Place cooked rice in center of bowl or plate. Add a scoop of the shrimp etouffee around rice. Garnish with parsley and green onions.