Heat the pot
Set the Instant Pot to Sauté, then add the oil or butter.
Brown the chicken
Season the chicken thighs with salt and black pepper. Brown them for 3 to 4 minutes on each side, then transfer them to a plate.
Cook the mushrooms
Add the sliced mushrooms to the pot and cook briefly until they start to soften. Stir in the garlic and dried thyme or mixed herbs, then cook for another 30 seconds.
Deglaze the pot
Pour in the chicken stock and scrape the base of the pot well to lift up any browned bits.
Add potatoes and chicken
Add the potatoes to the pot, then sit the browned chicken thighs on top.
Pressure cook
Lock the lid and cook on High Pressure for 10 minutes.
Release the pressure
Leave the pot to release pressure naturally for 10 minutes, then carefully release any remaining pressure.
Remove the chicken and potatoes
Lift out the chicken and potatoes and keep them warm while you finish the sauce.
Make the creamy sauce
Set the Instant Pot back to Sauté. Stir the double cream or cream cheese into the mushroom sauce, then add the Parmesan.
Thicken and season
Simmer for 2 to 4 minutes, until the sauce has thickened slightly. Taste and adjust with salt and black pepper.
Return and serve
Return the chicken and potatoes to the pot, spoon the creamy mushroom sauce over everything and finish with chopped parsley if using.