Using the Sauté function, heat bacon grease or canola oil. Brown half of the Andouille sausage. Flip and brown on the other side, about three minutes each side. Remove from Instant Pot. Repeat with the other half and remove from Instant Pot.
Add the celery, onion and green bell pepper to the Instant Pot. Cook until soft, about four to five minutes, stirring occasionally. Add garlic and cook from 30 seconds, stirring constantly.
Pour in chicken stock. Add browned Andouille sausage, ham hocks, red beans, Cajun seasoning, cayenne pepper (if using), thyme sprigs and bay leaves.
Put on the lid and turn the valve to seal. Cook on manual for 50 minutes.
When the time is up, allow to naturally release for at least 20 minutes. Remove the lid and turn again to the Sauté function.
In a small cup, stir together cornstarch and just enough cold water to make a slurry. Stir into the Instant Pot. Cook uncovered until it thickens, about 10 minutes.
Remove thyme and bay leaves and stir in apple cider vinegar. Add salt to taste if needed. You can also add even more Cajun seasoning or cayenne pepper if you'd like more heat. Serve on white rice with hot sauce and fresh parsley if desired. Enjoy!