Spray your slow cooker with a nonstick spray. Season the chicken with salt and black pepper and place on the bottom of the slow cooker. Top with mushrooms, onion, garlic, Italian seasoning, salt, and black pepper. Pour Marsala wine and chicken broth on top. Cover and cook for the stew on low for 5-6 hours.
After 5-6 hours, remove the chicken from the slow cooker. Cut it into 1-inch bite-sized pieces and set aside.
In a small bowl, whisk together the water and cornstarch until the cornstarch has dissolved. Whisk it into the slow cooker and add the chicken. Cover and continue to slow cook for an additional 20 minutes or until the sauce has thickened. Adjust seasoning to taste and keep warm until you are ready to serve.
Serve the chicken marsala stew on top of a bed of rice, pasta or mashed potatoes. Garnish with fresh parsley and enjoy!