Lightly grease a 6-quart slow-cooker with cooking-spray & put aside.
Season chicken-breasts all about with garlic-powder, dried-basil, thyme, paprika, salt, and pepper.
Heat olive-oil in a skillet over medium-high-heat. Then add prepared chicken-breasts into the hot-oil & cook about 3minutes each side, or tlil browned.
Transfer chicken to slow-cooker & top with garlic and mushrooms. Then pour skillet medium-high-heat then add the marsala wine, cook for a few minute, scraping-up all of the browned bits from the bottom of the skillet.
Pour wine over chicken & mushrooms. Then close with a lid and cook on LOW for 4-5hours or on HIGH for 2-3hours. Chicken is done when internal temperature reaches 165°F.
Remove chicken-breasts from slow-cooker & put aside on a plate.
Combine water & cornstarch in a mug or a smal- mixing-bowl and whisk till incorporated. Afterward, whisk the slurry to the wine-sauce.
Add heavy-cream into the wine-sauce & whisk till blended. Then return chicken into the slow-cooker, cover with a lid & cook on HIGH for 20minutes, or till sauce has thickened.
Transfer chicken from slow-cooker to dishes, top with mushroom-sauce and garnish with fresh-parsley. Serve immediately.