Layer Ingredients: Place seasoned chicken breasts in the bottom of a greased slow cooker. Top with sliced mushrooms and minced garlic.
Add Liquids: Pour the dry Marsala wine and chicken broth over the chicken and vegetables.
Slow Cook: Cover and cook on LOW for 4 to 5 hours until the chicken is tender and reaches 165 degrees Fahrenheit.
Thicken Sauce: Remove chicken to a plate. Whisk heavy cream and cornstarch together, then stir into the slow cooker liquid. Cook uncovered on HIGH for 20 minutes until thickened, then return chicken to the sauce.