In a heavy skillet (preferably cast-iron) cook oil and flour over moderately low heat, stirring constantly with a flat-edged metal or wooden spatula, until roux is the dark reddish-brown color of chestnut shells, about 45 minutes. (The roux may be made 1 week ahead, cooled completely, and chilled, covered, in a glass or stainless-steel bowl. Reheat roux in skillet over moderately low heat, stirring, before proceeding.) Stir in onions, bell pepper, and celery and cook, stirring occasionally, until vegetables are softened.
In a 10 to 12-quart heavy kettle bring 4 quarts water to a boil and add carrot tops and assorted greens. Simmer greens, stirring occasionally, until just tender, about 15 minutes. Transfer greens with tongs to a large shallow baking dish, letting excess cooking liquid drip into kettle and reserving all cooking liquid. Cool greens until they can be handled and coarsely chop.
Stir beans and remaining 3 quarts water into reserved cooking liquid and simmer beans, covered partially, until tender, about 1 hour. (Older beans may take longer to cook.) Add roux mixture by large spoonfuls, stirring well after each addition, and stir in greens, garlic, and salt and pepper, to taste. Simmer gumbo, partially covered, stirring occasionally, 30 minutes. Add Worcestershire and hot sauce, to taste. (Gumbo may be made 2 days ahead, cooled completely, uncovered, and chilled, covered.)
Stir in file powder, if using, and remove from the heat.
Serve gumbo ladled over rice in large soup plates.
Recipe courtesy of Gourmet Magazine