Slice chicken breasts in half lengthwise. Flatten each piece between 2 sheets of waxed paper or plastic wrap with a meat mallet to about ¼-inch thickness. Dredge each flattened piece in the flour.
In the same pan you cooked the mushrooms in, add 1 tbs of the butter and heat over medium-high heat.

Add 2 chicken breast pieces and brown well on all sides. Remove and set aside.
Repeat the browning process 2 more times (using a tbs of butter with each shift)–you have to do this in shifts so the chicken is not crowded in the pan and browns evenly. Don’t drain the drippings, you need it for the marsala sauce.

Preheat oven to 450 degrees F. Lightly grease a 13×9-inch baking pan.
Add the chicken breasts to the prepared baking pan, overlapping each piece slightly. Sprinkle evenly with the mushrooms.