Make a dark roux: Heat oil in a large soup pot over high heat. When oil begins to smoke, whisk in flour. Continue to whisk constantly until mixture is a rich brown color. Be careful not to produce specs of black. Roux must remain an even color throughout process. If specks appear you must start over. When roux is mahogany colored, add onions, celery and bell pepper.
Stir mixture until onions begin to brown. Add garlic. Slowly pour in stock, whisking constantly to prevent lumps from forming. Add Worcestershire, hot sauce, cayenne, salt and pepper to taste.
Bring to a boil, reduce heat to medium and simmer until gumbo is reduced to half a gallon. If it reduces too fast, add more stock or water. Gumbo should cook long enough for roux flavor to mellow.
Stir in shrimp, chicken, sausage, crab, green onions, parsley, bay leaves, thyme and basil. Taste and adjust seasonings, if necessary. Sprinkle in file powder and cook, stirring, 2 minutes more. Remove bay leaves before serving.
Yield: 4 to 6 servings