As Justin Wilson used to say, "firs' ya gotta make a roux, you know dat!" If you don't "know dat" you should realize to make a proper gumbo, sauce piquant, etouffée, or crawfish stew, for example, you need to know how to make a Roux. Pronounced "roo," it is the result of cooking nearly equal amounts of oil and flour together until it is nice and brown. Different shades of brown are achieved by different cooking times and temperatures. The longer the cooking time, the darker the color and flavor, and the less thickening quality it has. Usually cooked in a cast iron pot or skillet, oil can be substituted with lard, shortening, or animal fat. Most of the old Cajuns insisted on using lard.
Although both descended from French kitchens, Cajun rouxs are usually darker in color, and are used more for flavor than as a thickening agent. Creole rouxs are used more to thicken and tend to be lighter in color, like a béchamel sauce. They often use butter instead of oil for a more savory flavor.
For most recipes, I will use a dark roux. Somewhere between the color of milk chocolate and dark chocolate. This takes some time. Etouffées, for instance, use a lighter roux, but this varies from dish to dish.
For a standard roux, we will use 1 cup of vegetable oil and 1½ cup of flour and a cast iron skillet or dutch oven. If you make it in a skillet you'll probably need to transfer it to another, larger vessel after making it. Heat the oil in the skillet on MED, whisk in flour to avoid any lumps. After smoothing out all the lumps, I use a heat-proof, silicon, flat-end spatula to stir the roux, you want to make sure it doesn't stick to the bottom of the pot, and this silicon spatula gets in all those little corners. Most cooks prefer using a wooden spoon.
Don't go off and leave it. With the spatula, completely brush/scrape the roux off the bottom and sides, wait a few seconds, letting it brown a bit, and then repeat. Keep it moving, don't let it burn. If black specks appear, throw it out and start again. Keep stirring until it is the desired color you need, take it off the stove eye, and continue with your recipe.
Roux can get to upwards of 500º and can burn you badly. Chef Paul Prudhomme calls it Cajun Napalm. Be careful and don't burn it, or yourself. Do a few trial runs before you plan a big dish. If you are making it to put in a recipe later, transfer it to a heat-resistant container, as it will continue cooking if you leave it in the skillet. It is best to make it in the pot or skillet you will be using for the particular dish. Most recipes will require you to sauté the *"Holy Trinity" in the roux. If so, put the onions in first, to wilt them for a few minutes, then add the remaining celery and green peppers. This picture shows a roux after the onions have been added.
Adding the "Trinity" will stop the cooking process, so make sure you have it where you want it (color-wise) before adding the onions. Oh, the roux will actually turn a darker reddish color after adding the onions, this is from the caramelization of the sugar in the onions. The smell is almost irresistible at this moment. If you wanna cook cajun food, you gotta make a roux.
* The Holy Trinity is diced onions, celery, and green bell pepper.