Season the chicken with salt, pepper, garlic powder, and thyme, then place it in the slow cooker.
Add the mushrooms and garlic over the chicken.
Pour in the Marsala wine and chicken broth, ensuring the chicken is mostly submerged.
Cover and cook on LOW for 5-6 hours or on HIGH for 2.5-3 hours until the chicken is tender.
Mix the cornstarch and water in a small bowl, then stir it into the slow cooker along with the heavy cream and butter.
Cook for another 20-30 minutes on HIGH, allowing the sauce to thicken.
Serve warm, garnished with fresh parsley, over mashed potatoes, pasta, or rice.