Heat olive oil in a large skillet over medium-high heat. Dredge chicken breasts in flour, shaking off excess. Sear until golden brown on each side, about 3-4 minutes per side. Remove and set aside.

In the same skillet, add butter and mushrooms. Sauté until mushrooms release moisture and begin to brown, about 5 minutes. They should emit a rich, earthy aroma.

Pour Marsala wine into the skillet, scraping up browned bits from the bottom. Allow the wine to reduce by half, intensifying its sweet aroma and flavor.

Add chicken stock and bring to a simmer. Return chicken to the skillet, covering it with sauce. Lower heat and cook until chicken is cooked through, about 10 minutes.

Stir cornstarch slurry into the skillet, stirring continuously to avoid lumps. Simmer until the sauce thickens and coats the back of a spoon, about 2 minutes.

Season the sauce with salt and pepper to taste. Ensure the flavors are balanced, with no single element overpowering.

Allow chicken to rest in the sauce off the heat for a few minutes. This step helps the flavors meld and the chicken remain juicy.

Serve Chicken Marsala hot, garnished with fresh parsley if desired. Pair with your choice of side to complete the meal.