Generously season the chicken thighs with sea salt and black pepper.
Select the "Sauté-More" setting on the Instant Pot. Add the avocado oil to the pot and heat until the oil it is shimmering. Working in batches, add the chicken thighs to the pot in a single layer. Brown the chicken on both sides, about 3-4 minutes per side, then transfer it to a plate and set aside.
Add the onion and celery to the pot and sauté until the onion is tender and translucent, about 5-6 minutes.
Add the minced garlic to the Instant Pot and sauté until fragrant, about 30 seconds.
Pour one cup of chicken stock into the pot, then mix in the Dijon mustard. Bring the liquid to a simmer and scrape up and browned bits off the pot with a wooden spoon.
Add the chicken thighs back to the Instant Pot along with any juices from the plate. Layer the turnips, carrots, and mushrooms on top of the chicken, then sprinkle in the rosemary, thyme, 1/2 teaspoon of sea salt, and 1/4 teaspoon of black pepper. Stuff the kale leaves on top, then pour in the remaining 2 cups of chicken stock.
Lock the lid, select "Pressure Cook-High" on the Instant Pot, and set the timer for 12 minutes.
When the timer goes off, allow the pressure to naturally release for 5 minutes, then release the valve and remove the lid. Pick out the chicken thighs, shred or roughly chop them, and return them to the pot.
To thicken the stew, ladle out 3/4 cup of liquid from the Instant Pot into a medium sized bowl. Sprinkle in the xanthan gum, then whisk well to combine. Pour the slurry back into the Instant Pot and select the "Sauté-More" setting. Heat, stirring occasionally until thickened, about 3-4 minutes.
Season the stew with more sea salt and black pepper to taste, then ladle it into bowls and serve.