Adapted from Joy of Cooking, and various allrecipes.com recommendations, also, I made this bascially for one person, if you are cooking for more, buy more chicken and use much more sauce!
For the chicken preparation:
2 chicken breasts, pounded thin
1/4 cup (probably less) flour
salt & ground black pepper
1 tablespoon olive oil
Season chicken with salt & pepper, dredge in flour, and shake off excess. Heat olive oil in saute pan, add chicken breasts and cook through, only flip once to keep the coating intact. Set aside on paper towels while cooking sauce.
For the Marsala preparation:
1/2 small onion, diced
1/2 cup sliced mushrooms, I used crimini
1/2 cup Marsala wine
1 tablespoon butter
1 tablespoon cornstarch
Add onion to the same saute pan and cook until softened. Add mushrooms and cook until mushrooms have released their water. Pour in Marsala and be sure to scrape up any bits that have stuck to the pan, bring to a boil, whisk in butter and cornstarch. Stir occasionally until sauce has reduced down to desired thickness. Season with salt & pepper to taste.