Season chicken breasts with salt and pepper to taste.
Turn the Instant Pot ON and select "sauté" mode, adjust heat to “more”. When hot, add oil. Add halved mushrooms and cook for 5 minutes, or until soft. Stir in diced onion and garlic and cook for 3 minutes. Season with salt and pepper to taste.
Add Madeira wine and beef stock to the pot. Mix to combine. Let it simmer for 5 minutes. Lay chicken on top of the mushrooms. Secure the lid to the instant pot. Close pressure release valve. Select "pressure cook" and adjust cooking time to 8 minutes.
When the time is over, carefully turn the valve to "Venting," to quickly release the pressure. Once the float valve has dropped, remove the lid. Press "cancel". Then use a slotted spoon to transfer chicken to a plate. Cover loosely with foil to keep it warm.
Select "sauté" adjust to normal. Add the cornstarch and water mixture; mix to combine. Cook for about 3 minutes, or until the sauce is slightly thickened, stirring occasionally. Stir in the butter until melted and incorporated into the sauce. Taste and adjust seasoning to your liking adding more salt and/or pepper, if necessary. Return chicken back to the pot and turn it to coat in the sauce.
Garnish with fresh chopped parsley, if desired. Serve hot.