In a large stockpot, add the canola oil and heat on medium. When the oil is hot, add half the andouille sausage and cook until browned, about two to three minutes. Flip and brown on the other side. Repeat with the other half of the sausage.
Turn heat down to medium-low. Add the duck breasts skin side down. Cook until the fat renders, about 15-20 minutes. Remove the duck breasts and repeat with the legs and reserved extra skin pieces. Measure out the rendered duck fat so that you have ½ cup. If you have extra duck fat, store in an airtight container in the refrigerator for future use.
Turn the heat to medium. Add the ½ cup duck fat back to the pot and whisk in the flour. This is the roux. Whisk continuously until the roux turns the color of melted chocolate. This should take about an hour, depending on your heat. As it cooks, you may need to reduce the heat to medium-low if it starts to smoke and get too hot. You don't want the heat too high to make the roux cook faster because it could burn.
Once the roux is ready, stir in the celery, green pepper and onion and cook for about five minutes. Stir the vegetables often. Add garlic and cook for about 30 seconds.
Slowly stir in the chicken stock or water with Better Than Bouillon. Add in the 2 tablespoons Cajun seasoning, bay leaves, Worcestershire sauce, hot sauce and sausage. Remove the skin from the duck and also add that. If desired, add the reserved duck neck and spine. Bring to a boil over medium-high heat and then reduce to a simmer over medium-low heat. Cover and simmer for about an hour and 20 minutes, stir occasionally.
Remove the duck. Discard the spine and neck if using. Using two forks, shred the meat off the bones. Add the shredded meat back to the pot and stir in the filé powder and parsley.
Taste the gumbo. If it needs more Cajun seasoning, add a little at a time until it suits your taste. If you'd like more heat, you can add cayenne pepper to your desired heat level. Keep in mind the flavors will bolden if you refrigerate it overnight. Serve with a scoop of white rice and additional hot sauce if desired. Enjoy!