Remove the cooked chicken from the pan and place it on a platter, keeping it warm while you prepare the sauce. Turn the heat to medium high and add the 1 cup of mushrooms, sautéing them until they're cooked. With the mushrooms still in the pan, deglaze with the 3/4 cup Marsala wine, stirring to scrape up all the browned bits from the bottom of the pan.
Turn the heat to medium low and continue to cook the sauce, stirring frequently, until it has thickened slightly. Taste the sauce and add more wine or butter if you think it needs it. If the sauce doesn't thicken as much as you'd like, whisk in a little bit of flour and continue to stir until it reaches your desired consistency. Once the sauce is ready, add the chicken back to the pan and turn to coat it.