Start by cleaning the chicken, trim all the visible fat off.
Season your chicken on both sides with salt & pepper.
Then turn on the saute' function, add 2 tablespoons of your butter and let the butter melt. As it is melting, add your chicken.
Brown both sides, just to get a nice color on the chicken, then remove the chicken and place it on a plate, for later use.
Then add 2 tablespoons of the butter, and the mushrooms, thyme and cook until the mushrooms are browned, about 5 minutes, mix in the garlic. Then mix in the flour, continue to stir until all well blended about 2 minutes more.
Stir in the Marsala wine and chicken broth/stock, mix well, place the chicken breasts on top of the mushroom mixture.
Place your lid on to the pressure cooker (Instant Pot) and set the high pressure setting for 2 minutes, making sure that the vent is sealed.
When the time is up, do a quick release. Remove the chicken, and place on a plate, then hit the saute' button again and continue to boil, until the mixture is slightly reduced and thickened.
Serve over rice or pasta.