Lightly spray slow cooker with cooking spray. Place oil in the bottom of the slow cooker and add the garlic. Salt and pepper the chicken breasts and place them on top of the garlic and oil. Place the mushrooms over the chicken and pour in the Marsala wine.
cook on low for 5 – 6 hours or until chicken is done. Remove the chicken to a plate and cover with aluminum foil to keep warm.
In a small bowl mix together the cornstarch and water until cornstarch is dissolved. Stir the mixture into the chicken juices, set the slow cooker on high, and continue cooking until mixture is thickened into a gravy. Add chicken back into the slow cooker and cook until heated through.
Serve chicken topped with the gravy, and garnished with the parsley.