In a large enameled cast-iron casserole, stir the remaining 2/3 cup of vegetable oil with the flour until smooth. Cook the oil and flour over moderately low heat, stirring constantly, until a deep brown roux forms, about 25 minutes. Add the garlic, scallion whites, celery, onions, green peppers and finely chopped andouille sausage. Cook over low heat, stirring occasionally, until the vegetables soften, about 15 minutes. Add the thyme, cayenne, 1 teaspoon of salt and 1/4 teaspoon of pepper and cook, stirring, for 2 minutes. Add the remaining 1 tablespoon of tomato paste and cook, stirring, for 1 minute. Gradually whisk in the reserved stock. Simmer over low heat for 1 hour, skimming occasionally.