Turn the Instant Pot saute setting on and melt the butter.
Add the diced onions and sliced celery to the Instant Pot. Saute for about 3 minutes or until the onion starts to turn translucent.
Add the sliced carrots, salt and ground pepper to the Instant Pot. Saute for 1 or 2 minutes longer.
Add the boneless skinless chicken, sliced portabella mushrooms, wild rice, fresh thyme, fresh rosemary, bay leaf and chicken broth to the pot.
Place the lid on the Instant Pot securing it and setting the valve to the sealing position.
Press the manual button and set the timer for 15 minutes.
Once the pressure cooker is done cooking allow the steam to naturally release for 10 minutes.
Carefully remove the lid of the Instant Pot allowing the steam to escape from the back side away from your body.
Remove the cooked chicken from the Instant Pot and place it in a bowl. Use two forks to shred the meat. Add the shredded chicken back to the Instant Pot.
In a small bowl mix the water with the cornstarch until lump free. Add the slurry of cornstarch and water (a gluten-free thickener) to the Instant Pot and mix until combined.
Turn the Instant Pot to the saute setting and bring to a boil, stirring. Boil until thickened, about 2 minutes.
Makes about 8 servings.