Pour 1 cup vegetable oil or canola oil into a large pot with a heavy bottom.
Heat oil on medium-high heat.
Once the oil is hot, add 1 cup flour. The flour will sizzle when you add it. Stir.
Reduce heat to low-medium (closer to medium). Cook roux stirring slowly and consistently. Make sure to scrape the sides and bottom of the pan while you stir.
The roux will begin to change colors as it cooks, and you will begin to smell a nutty aroma. Keep stirring.
In about 15-20 minutes, your roux should be getting darker. At this time, you may want to adjust your heat to a lower setting to prevent burning.
Once it is the color of dark chocolate (Usually within 25-30 minutes) you will want to either add your holy trinity (bell pepper, onion, and celery) and begin cooking your gumbo or remove the roux from heat and let cool.