Add chicken breasts to Ziploc bag. Pound to 1/4 inch thick using a kitchen mallet. Pat chicken breast dry with paper towels and season on both sides with salt and pepper.
Press Sauté on Instant Pot and add oil when the display reads HOT.
Add garlic and mushrooms. Cook and stir for 2-3 minutes.
Deglaze with chicken broth.
Stir in marsala wine, lemon juice, and oregano.
Add chicken breasts on top of the mushrooms and press down until they are submerged by the sauce.
Secure the Instant Pot lid and turn valve to sealing.
Press Manual/Pressure Cook and set time to 5 minutes.
When cook time ends, quick release pressure. Press Cancel button.
Move chicken breasts to a plate and press Sauté button.
Add heavy whipping cream. Stir well. Scoop out some sauce into a small bowl, add xanthan gum and mix well.
Pour into the pot and stir until sauce thickens, press cancel to stop sauté mode.
Add chicken back in and let marinate in sauce for one or two minutes.