Set your cooker to sauté and add the olive oil to heat. Sear chicken breast side down then flip and sear the other side. Remove and set aside.
Add butter and heat until foaming. Add carrot, celery, onion, mushrooms and garlic and sauté. Add all remaining ingredients except the parsley. Put back the chicken and pan juices in the centre so the liquid and veg is surrounding the chicken.
Put on the lid and pressure cook 30 minutes. Allow it to cool enough for you to take off the lid. Remove the chicken and set aside. Set to sauté again for 20 minutes to reduce the liquid.
Add your teaspoon of xanthan gum if you want a thicker sauce, sprinkling in a tiny bit at a time while constantly whisking it in. Arrange your chicken, veg and sauce on a serving dish or on plates. Scatter with parsley to garnish.