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, beer, and Essence. Stir until the roux mixture and stock are well combined. Bring to a boil, then reduce heat to a simmer. Cook, uncovered and stirring occasionally, for 2 1/2 to 3 hours, until the ham hocks are very tender, skimming any fat that rises to the surface.

Remove the bay leaves and ham hocks and set aside until the ham hocks are cool enough to handle. When the hocks have cooled enough to handle, shred the meat from the hocks and place the meat back into the gumbo. Discard bones and skin. Remove the gumbo from the heat and stir in the parsley, green onions, and file powder. Serve the gumbo in deep bowls with the rice.