In a shallow bowl, mix the flour with a generous amount of salt, pepper, and chopped parsley leaves.
Heat a skillet over medium-high heat and add approximately 2 tablespoons of butter. Bring to a bubble, but not quite browning.
Dredge the flattened chicken in the flour mixture, shaking off any excess, then transfer it directly to the skillet (if using a smaller skillet where the chicken won't fit side-by-side, do one breast at a time). Brown for 5 minutes per side, until golden and crispy.
Remove the chicken from the pan and keep warm. Leave the drippings in the pan.
If using bacon, toss it into the skillet while you slice the mushrooms. When the bacon is crisp, remove it from the pan and crumble it, but leave the grease in the pan.
Add the sliced mushrooms to the pan and sauté until they have absorbed enough of the liquid and have started to brown.
Sprinkle the 2 tablespoons of flour over the mushrooms and stir constantly until they are coated in flour.
Pour in the Marsala wine, chicken broth, and lemon juice, and stir until smooth. Let the mixture simmer for a few minutes to thicken the sauce.
Plate the chicken and spoon the mushroom sauce over it. Sprinkle with crumbled bacon, if desired, and serve.