Lightly butter the interior of the slow cooker inner pot or spray with nonstick cooking spray.

Pat the chicken breasts with paper towels to dry and sprinkle with salt, pepper, and garlic powder.

Arrange the chicken in the slow cooker and add the mushrooms, scallions, chicken stock, and wine.

Cover and cook for 5 hours. If your slow cooker cooks fast, check for doneness after about 4 hours. The chicken must be cooked to a temperature of at least 165 F. Overcooked chicken breasts can be dry.

With a slotted spoon, transfer the chicken to a plate and set aside.

Combine the cornstarch and water and whisk until well blended. Stir the cornstarch mixture into the slow cooker liquids until well blended. Add the chicken back to the slow cooker and turn the setting to high. Cover and cook for about 8 to 10 minutes longer, or until thickened.

Serve chicken Marsala with hot cooked rice, angel hair pasta, or potatoes.