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. In a hot saute pan, add the duck breast, fat side down, render for about 4 to 5 minutes, or until the fat is crispy. Turn the breast over and continue to cook for 3 to 4 minutes. Remove the duck from the pan and cool. Slice the breast into 1/4-inch slices. Add the onions, celery, and bell peppers and continue to stir for 4 to 5 minutes, or until wilted. Add the mushrooms and continue to saute for 2 minutes. Add the sausage, salt, cayenne, and bay leaves. Continue to stir for 3 to 4 minutes.. Stir the stock into the roux mixture until well combined. Bring to a boil, then reduce heat to medium-low. Cook, uncovered, stirring occasionally, for 2 1/2 hours. Skim off any fat that rises to the surface. Stir in the sliced duck, crawfish and Rustic Rub. Continue to simmer for 30 minutes. Remove from the heat. Stir in the parsley, green onions, and file powder. Remove the bay leaves and serve in deep bowls.
Combine all ingredients and store in an air-tight container.