Combine the oil and flour in a large cast-iron or enameled cast-iron Dutch oven, over medium heat. Stirring slowly and constantly for 20 to 25 minutes, make a dark brown roux, the color of chocolate. Add the onions, celery, and bell peppers and continue to stir for 4 to 5 minutes, or until wilted. Add the cabbage and continue to saute for 2 minutes. Add the ham hocks, salt, cayenne, and bay leaves. Continue to stir for 3 to 4 minutes. Add the stock and Essence. Stir until the roux mixture and stock are well combined. Bring to a boil, then reduce heat to medium to low. Cook, uncovered, stirring occasionally, for 2 1/2 hours. Skim off any fat that rises to the surface. Continue to simmer for 30 minutes. Remove from the heat . Stir in the parsley, green onions, and file powder. Remove the bay leaves and ham hocks. Shred the meat from the hocks and place the meat back into the gumbo. Serve in deep bowls with the rice.
Combine all ingredients thoroughly.