Sauté the Chicken: Set the Instant Pot to Sauté (High/More setting) and heat the olive oil for about 3 minutes. In batches, sear the chicken for 30 seconds per side, moving it constantly to prevent sticking. Remove and set aside.
Cook the Vegetables: Add the butter and onions, scraping up any browned bits from the bottom. Stir in mushrooms and garlic, sautéing for about 3 minutes until softened.
Brown the Sausage: Add the sliced sausage and cook for 2 minutes, stirring occasionally.
Deglaze with Marsala Wine: Pour in the Marsala wine, bringing it to a simmer while scraping up any browned bits. Stir in chicken broth, rosemary, thyme, Italian seasoning, and seasoned salt. Place the chicken back into the pot on top of the mixture.
Pressure Cook: Secure the lid, set the valve to Sealing, and select Pressure Cook (High) for 6 minutes. Once done, quick release the pressure and transfer the chicken to a serving dish to rest.
Prepare the Slurry & Add Peppers: In a small bowl, mix cornstarch with 2½ tablespoons of cold water to make a slurry. Stir in roasted red peppers and heavy cream to the pot.
Finish the Sauce: Select Sauté (High/More setting) again, and once the liquid starts to bubble, add the cornstarch slurry and Parmesan cheese. Stir well, simmering for 30 seconds. Turn off the pot and let the sauce sit for 5 minutes to thicken.
Serve & Enjoy: Pour the sauce over the chicken and serve over linguine, fettuccine, or capellini for a complete meal.