Place the Instant Pot on the saute function. When the pot is hot, add the olive oil.
Add the chicken breasts to the pot. Season the breasts with the chicken seasoning, salt, and pepper. Sear each side of the breasts for a couple of minutes.
Remove the chicken from the pot. Add the garlic and shallots. (Use additional olive oil here if necessary). Cook for 1-2 minutes until fragrant.
Add the mushrooms. Cook for 1-2 minutes.
Add the chicken broth to the pot. Deglaze the pot by using the liquid to scrape up the brown bits from the chicken. There is so much flavor in this process.
Add the marsala wine and Better Than Bouillon to the pot and stir. Add the chicken breasts back to the pot.
Seal the pot with the lid. Set the cooking time to Manual High-Pressure Cooking for 5 minutes.
When the pot indicates it is finished, do not perform a quick release. Allow the steam to release naturally for 5 minutes.
Combine the cornstarch and water in a small bowl. Create a slurry by mixing the two.
Open the pot. Place the pot on the saute function. Add the slurry and heavy cream. Stir. This will thicken the sauce. If the sauce does not thicken enough for your preference, create an additional slurry using cornstarch and water.
When the sauce has thickened, remove the chicken and sauce from the pot. Serve.