Make the roux: mix oil and flour in a large heavy saucepan over low heat, Whisk flour into the oil to form a paste. Continue cooking over low heat and whisk continuously, until the mixture turns a caramel color and gives off a nutty aroma, about 15 to 20 min.
To the roux, add the onion, green pepper, celery, and garlic and cook over low heat about 5 minutes, until the vegetables are limp.
Add the black pepper, white pepper, cayenne pepper, Cajun seasoning, green onions, parsley, and hot sauce to taste. Add clam juice and the tomatoes with their juice, stir to blend. Add the salt, starting with 1 teaspoon then add more if needed. Bring the mixture to a boil, reduce the heat to low and simmer for 10 to 15 minutes.
Add shrimp and stir. It will take about 3 minutes for shrimp to cook, don't overcook. Remove from heat. Add the butter and stir, the heat from the dish will melt the butter.
Transfer the etouffee to a tureen, serving bowl, or if you prefer, over rice. Garnish with the green onions.