Heat a skillet over medium high heat and add 2 tablespoons of butter.
Add the mushrooms to the skillet and cook, stirring often, for 10 minutes.
Add onions and garlic to the mushrooms and continue to cook for 5 minutes until mushrooms and onions are tender and cooked through. Once cooked set mushrooms and onions aside.
Add remaining 2 tablespoons of butter and 2 tablespoons of flour. Stir and cook for one minute.
Add Marsala wine to the skillet to deglaze the pan, using a wooden spoon to scrape up any browned bits off the bottom of the pan.
Add beef broth and turn the heat up bringing the mixture to a boil. Boil for about 5 minutes, stirring constantly, and then reduce to a simmer. Keep cooking until mixture is reduced by half and the sauce coats the back of a spoon. Approximately 10 minutes.
Add the cooked mushrooms and onions to the sauce stirring together.
Place the sliced turkey in the sauce and continue to cook for 3-5 minutes, or until warmed through.
Serve over mashed potatoes or vegetables.