Prepare the Chicken: Dredge the chicken fillets in the seasoned flour mixture, ensuring they are evenly coated. Set aside.
Sauté the Chicken: Set the Instant Pot to Sauté mode on High. Heat the olive oil and 1 tablespoon of butter until melted. In batches, lightly sear the chicken for about 1 minute per side until golden brown. Transfer to a plate.
Sauté the Aromatics: Melt the remaining butter in the pot without wiping it clean. Add the shallot and garlic, sautéing for about 2 minutes while scraping up any browned bits.
Cook the Mushrooms: Stir in the sliced mushrooms and continue to sauté for another 2 minutes until softened.
Deglaze with Wine & Add Chicken: Pour in the Marsala wine and scrape the bottom of the pot to release any stuck bits. Add the chicken broth and return the chicken to the pot.
Pressure Cook: Secure the lid, set the valve to Sealing, and select Manual or Pressure Cook on High Pressure for 8 minutes. Once done, perform a quick release.
Prepare Cornstarch Slurry: In a small bowl, mix the cornstarch with 1½ tablespoons of cold water to form a smooth slurry.
Thicken the Sauce: Remove the chicken and set aside. Keep the Instant Pot on Sauté mode and bring the sauce to a simmer.
Finish the Sauce: Once bubbling, stir in the cornstarch slurry and cook until the sauce thickens, then turn off the Instant Pot.
Serve: Pour the sauce over the chicken and serve hot.