Begin by heating olive oil in a large pan over medium-high heat. Season both sides of the chicken breasts with salt and pepper before placing them in the pan. Sauté the chicken for 4 to 5 minutes on each side, or until golden brown.
Transfer the browned chicken breasts to the slow cooker. Add minced garlic, Marsala wine, chicken broth, and sliced mushrooms. Cover and set the slow cooker to cook on the LOW setting for 5 hours.
Once the cooking time is complete, remove the chicken from the slow cooker. In a separate bowl, whisk together the cornstarch with 1/3 cup of cold water. Stir the cornstarch mixture into the slow cooker, combining it with the sauce.
Return the chicken to the slow cooker, then turn the heat to HIGH. Allow the sauce to cook for an additional 30 minutes, or until it thickens.
Stir in the heavy cream, adjusting the seasoning with salt and pepper as needed. Finish by sprinkling with fresh parsley and serve immediately.
