Lightly grease a 6-quart slow cooker with cooking spray.
Season the chicken breasts with garlic powder, basil, thyme, paprika, salt, and pepper.
Heat olive oil in a skillet over medium-high heat and brown the chicken for about 3 minutes per side.
Transfer the chicken to the slow cooker, top with mushrooms and minced garlic.
Deglaze the skillet with marsala wine, scraping up the browned bits, and pour over the chicken.
Cook on LOW for 4 to 5 hours, or HIGH for 2 to 3 hours.
Remove chicken and whisk cornstarch and water into the sauce. Add cream, adjust seasoning, and return chicken to thicken the sauce on HIGH for 20 minutes.
Serve garnished with fresh parsley.