Heat the olive oil on saute mode of the Instant Pot. Brown both sides of the chicken breasts and toss in the mushrooms and saute for 3 to 4 minutes. Press Cancel.
Pour in the chicken stock and marsala cooking white and sprinkle in the garlic powder and minced onion. Close the lid.
Set to Sealing.
Cook on Manual, High Pressure, for 15 minutes.
Let naturally release. Once you open the lid, mix the cornstarch with equal parts water and then stir in the slurry to thicken the sauce. Leave on Keep Warm mode for a few minutes, until sauce thickens.
Cook the pasta or rice while it's cooking.
Serve Instant Pot Chicken Marsala over pasta or rice, with side salad or veggies.