Preheat a large, deep skillet or shallow pot over medium high heat.
Season both sides of the chicken with salt and pepper.
Add 2 Tablespoons of oil. When the oil is hot, add the chicken. Cook until golden brown, approximately 5-7 minutes. If too hot, reduce the heat.
Flip the chicken over and cook an additional 5 minutes until golden brown and also cooked all the way through. Add a little more oil if needed. If your chicken is browning too fast, reduce the heat to slow browning but continue cooking the chicken. The chicken is done when it is 165F in the thickest part.
Remove the chicken from the pan and set aside. If needed, add a little more oil so there is approximately 2 Tablespoons of oil in the pan. Heat until the oil is warm.
Add the mushrooms and cook until they soften and begin to release moisture, approximately 3 minutes. Add the garlic, stir, and cook for 30 seconds until fragrant.
Add the cornstarch to the mushroom mixture, stir, and let cook for 30 seconds. Add the marsala wine, scrape up bits from the bottom of the pan with the spoon, and simmer over medium heat until the sauce is thickened, approximately 5-8 minutes.
Stir the lemon juice into the sauce and add the butter. Stir until the butter is melted and incorporated.
Season the sauce as desired with additional salt, pepper, or lemon juice. Add the chicken back to the sauce and to heat through and coat with the sauce, if desired. Serve over pasta or with vegetables, rice, or potatoes. Garnish with fresh parsley if desired.
If your sauce is too thick, add some water or chicken stock. If too thin at any point, continue to cook over medium heat to thicken.It is best to use true Marsala wine and not cooking wine for the best flavor. This can be found at your liquor store.