Turn the Instant Pot to saute, add in 1 tablespoon of olive oil, and let heat.
While the oil is heating, season the chicken breasts generously with salt and pepper on each side. Add the seasoned chicken to the inner pot in a single layer (work in batches if needed) and sear the chicken until golden on each side. This will take about 2-3 minutes per side. Remove the browned chicken breasts from the inner pot and place them on a plate.
If the inner pot is dry, add 1-2 teaspoons of additional olive oil to the inner pot. Add in mushrooms and dried thyme and saute for 2-3 minutes to let the mushrooms soften. Add in the garlic and saute briefly (30-60 seconds), just to let the garlic toast.
Add in the Marsala wine and deglaze the pan, by scraping up any browned bits of chicken off the bottom of the pan. Turn the heat OFF on the Instant Pot.
Return the seared chicken, along with any accumulated juices back to the inner pot and then pour in the beef stock.
Place the lid on the pressure cooker, turn the valve to sealed, and set the cooking time using the pressure cook or manual button, to high pressure for 6 minutes for thin chicken breasts, 7 minutes for average size chicken breasts, and 8 minutes for larger chicken breasts. Once the cooking time has elapsed, let the pressure release naturally for at least 10 minutes (this helps to keep the chicken from getting tough).
Once pressure has been released, remove the lid and then remove the cooked chicken from the inner pot, place it on a clean plate or serving platter, and tent it with foil.
Hit cancel on the pressure cooker to turn off the keep warm function and then hit saute to bring the cooking liquid to a boil.
While the marsala sauce is coming to a bubble, combine the cornstarch with 2 tablespoons of water in a small bowl and whisk really well to fully dissolve the cornstarch. Add the cornstarch slurry to the inner pot, whisking constantly until the sauce has thickened--about 3-4 minutes. If desired, whisk in the butter and heavy cream and let heat through. Turn off the saute function.
Pour the sauce over the chicken and serve immediately.
Video
Notes
To determine cook time: Use 6 minutes for chicken breasts/thighs that are between 6 and 8 ounces each, 7 minutes for 8-9 ounce chicken breasts/thighs, and 8 minutes for 9-10 ounce chicken breasts/thighs (or really thick chicken). Add 1 minute of cooking time if you skip the step of searing the chicken. Marsala Wine: Be sure to use a good quality dry Marsala Wine for this Chicken Marsala recipe--not a cooking Marsala wine you find in the spice aisle of your grocery store.Beef Broth/Stock: I use beef broth to add extra flavor to the Chicken Marsala sauce, but you can use chicken stock or mushroom broth if you'd like.Add Cream: If you would like a creamy marsala sauce, like my stovetop recipe, whisk in up to ½ cup of heavy cream with the cornstarch slurry.Leftovers: Leftovers can be stored in the refrigerator for up to 3 days in an airtight container.