Put the chicken breasts on a cutting board and lay a piece of plastic wrap over them; pound with a flat meat mallet, until they are about 1/4-inch thick. You can also slice the chicken breasts in half or buy them thinly sliced.
Put the flour in a shallow platter and season with a fair amount of salt and pepper and garlic powder; mix with a fork to distribute evenly. Dredge both sides of the chicken breasts in the seasoned flour, shaking off the excess.
Turn the Instant Pot ON and select "saute" mode, adjust heat to more. When hot, add 2 tablespoons of oil and 2 tablespoons of butter. When melted, Add the 2 - 3 chicken breasts into the pot and fry on each side until golden, turning once. Remove the chicken to a large platter in a single layer to keep warm. Repeat with the rest of the chicken breasts.
Add the remaining oil and butter to the pot. When melted, add mushroom, shallots and garlic. Cook for 5 - 7 minutes, or until the mushrooms are lightly browned, stirring occasionally. Season with salt and black pepper to taste. Press "cancel".
Add Marsala wine and chicken stock to the pot. Mix to combine. Lay chicken on top of the mushrooms. Secure the lid to the instant pot. Close pressure release valve. Select "poultry" and adjust cooking time to 10 minutes.
When the time is over, let the pressure release on its own for 5 minutes. Then, carefully turn the valve to "Venting," to release any extra pressure that might still be in there. Then remove the lid. Press "cancel"
Transfer chicken to a serving plate and cover with foil to keep it hot, if desired.
Select "saute" adjust to normal. Add heavy cream; mix to combine. Cook for about 5 minutes, or until the sauce is slightly thickened, stirring occasionally. Place chicken back in the sauce and continue to simmer for about 1-2 minutes.
Garnish with fresh chopped parsley, if desired. Serve hot.