In a large heavy stockpot or cast iron skillet, combine flour and oil over low heat and cook, stirring constantly until mixture reaches a nice dark pecan-brown color. (Here is a great tutorial on making a roux, http://www.recipezaar.com/bb/viewtopic.zsp?t=225104&sid=378ee16fc9829f21e4454a60406c07ce.
Next add your onions, bell pepper and garlic to the pot. Let cook, stirring often, until veggies are tender. (If you used a skillet to cook your roux you will need to transfer this mixture to a large stockpot at this point.).
Add all remaining ingredients except rice, okra and file (pronounced Fee-lay).
Allow gumbo to come up to a boil, then reduce heat, cover and let cook for 1 to 3 hours.
It is my experience that the longer you let it simmer the better the flavor. Some folks around here even go so far as to cook gumbo a day ahead because they believe a good gumbo is at least 24 hours old.
About 1 hour before you want to serve the gumbo you can add the okra and file, and let the mixture return to a boil. Reduce heat again and let it continue to simmer until you are ready to serve.
Meanwhile, prepare rice according to the package directions.
Extra file should be placed in a small bowl and served alongside the gumbo for those who want to add some to their bowl. Gumbo file is a thickening agent that comes from ground sassafras leaves and gives gumbo it's distinctive flavor.