To begin, understand that the best Louisiana gumbo will use this traditional roux. There are other recipes that do not use this brown flour and oil mixture but this page is all about that wonderful roux. These specific instructions are all you need for success.
Use a heavy pot like cast iron. If you don't have cast iron any heavy bottom pot will work. It could be a skillet or deep pot just a long as it has a fairly heavy bottom. A thin bottom pot gets hot too fast, heats unevenly and will likely burn the flour. If you must use a pot having a thin bottom, be very careful. Use a low heat and stir rapidly non stop.
My favorite type of pot for making roux is cast iron. I like both the enameled cast iron pots and the seasoned non enameled pots from Lodge. This type of cookware is ideal for simmering gumbo and is perfect for making roux.
No type of cookware can rival the even heating, heat retention, durability and value of Lodge cast iron. Its legendary cooking performance keeps it on the list of kitchen essentials for great chefs and home kitchens alike. If you don't have a cast iron dutch oven then you must have one. It will become your favorite cookware.
Have your vegetables diced and ready. I like to cover the diced onions, green pepper and celery while the roux is cooking so they stay fresh. You will need a spatula or wire whisk and a large spoon by your cook top. I find that a large wire whisk gives the best control and is most easy to use. It will cause less splatter and reaches all corners of the pot which helps to ensure there are no burned areas. Measure your oil and flour and have that ready in advance.
At your cook top turn the heat on high, place your pot on the burner and
put in the oil and flour. Stir until this is mixed well. When the oil
and flour start to bubble stir constantly for 5 minutes then turn your
heat to down to medium high. Keep stirring.
In a few minutes the flour will start to darken. When you see a caramel color developing turn the heat down to medium. Keep stirring using your spatula or whisk taking care to scrape every part of the pot bottom. Don't stop. Relax. A traditional roux takes a little time.
The next image shows the roux beginning to turn color. This is the first stage and is your confirmation the roux is progressing well.
After a little more stirring you will see the flour getting browner. Keep stirring. Maintain the medium heat setting. A high heat will burn. Trust me on this. Your goal is to get the roux browned to the color of milk chocolate or an old copper penny. I have taught many people how to get a correct color. To help demonstrate the color stages I will have a jar of peanut butter out to compare to the roux to show phases of color. When the roux reaches a color of peanut butter it is about half way there.
Keep on stirring as the roux continues to darken. Here is where the wire whisk really performs for you.
The photo above is an example of a roux that is almost ready but it is not at the ideal color stage. It needs to be darker. However, if you were to stop here this roux would make a good Creole style gumbo commonly served in New Orleans. I call this color a dainty brown. To get the best Cajun style gumbo the color needs to be darker.
At this color stage I recommend a low heat setting. This allows you to maintain control of the darkening process without the risk of burning. You will probably see a slight amount of smoke rising from the pan and this is normal. Just turn on your vent to remove the smoke.
Keep going, keep stirring and soon enough your roux will start approaching that red brown or milk chocolate color stage that is so important. The photo above shows the color you want to achieve for the perfect traditional roux.
When your roux is red brown or the color of milk chocolate put in the diced vegetables. When the onions, green pepper and celery (known as the trinity) go into the roux there will be a lot of hissing and steaming. This is normal. Use your spoon to stir and blend in this mixture. Keep the heat on low. Keep stirring and scrape the bottom of the pot.
Soon the vegetables will start to get transparent and the roux will get
darker. Don't worry about the darker color at this point. This is due to
the moisture and sugar in the vegetables and does not mean the roux is
burning. In fact the entire mixture should be real dark after about 10
minutes of cooking on low heat.
If I am making a large size gumbo I will transfer the finished roux and trinity to a larger pot, the one I will cook in. Here is a picture of what that would look like.
At this point the traditional roux and vegetable mixture is ready. You can add your stock and seasonings here or if you want to finish the gumbo later you can turn off the heat and come back. This mixture will freeze well for future use.