Flatten the chicken breasts between two pieces of waxed paper with a meat pounder until thin. If you're not a gadget person like myself and don't own every kitchen utensil available including a meat pounder, try using the edge of a can of soup or a small sauté pan.It's important that you pound the breasts to an equal thickness. This is so they cook evenly and one breast doesn't cook faster then another.
Slice the mushrooms into thin slices and finely chop the parsley.
Add the butter to a fry pan / skillet, melt over medium heat, add the mushrooms and cook until lightly browned. It's hard to tell you exactly how long this will take since all stove tops are not alike.Depending on whether you are using a gas or electric or what make or model, will make a difference. On my household gas stove, it takes 8 - 12 minutes, but on my mom's electric stove, it takes longer.
Remove the mushrooms and reserve.
Add the oil to the pan and up the heat a little. I don't advise cooking at high until you get comfortable cooking at high heat. It means working a little faster and watching a little closer but you want the pan to be hot to pan fry the breasts properly.
While the oil is heating up, dredge each chicken breast in flour. What I like to do is put the flour in a shallow soup bowl or on a plate and using one hand, dip each breast into the flour and add to the pan.
Sauté the breasts for approximately 1-2 minute each side.
Remember the chicken breasts will cook faster because they are thinner. If you are preparing more than four breasts, it's important not to crowd the pan so they fry and do not steam.
Better to cook in batches than trying to cook them all at once.
Remove the breasts from the pan, reserve on a platter, and cover with tin foil to keep warm.
Add the Marsala wine to the pan to deglaze, scraping the small bits of chicken stuck to the pan.
(Remember, Marsala is a wine and contains alcohol so be careful when adding it to a hot pan. Better to remove the pan from the heat when deglazing any pan with wine.)
Simmer until slightly reduced, approximately 2 minutes, again depending on your stove top.
Add the reserved mushrooms, season with salt and pepper and parsley.
Plate the chicken breasts along with your sides and top with sauce.