Lightly grease a 6-quart slow cooker with cooking spray and set aside.
Season chicken breasts all around with garlic powder, dried basil, thyme, paprika, salt, and pepper.
Heat olive oil in a skillet over medium-high heat.
Add prepared chicken breasts to the hot oil and cook for 3 minutes per side, or until just browned.
Transfer chicken to slow cooker and top with mushrooms and garlic.
Return skillet over medium-high heat and add the marsala wine; cook for 1 minute, scraping up all the browned bits from the bottom of the skillet.
Pour wine over chicken and mushrooms.
Close with a lid and cook on LOW for 4 to 5 hours, or on HIGH for 2 to 3 hours. Chicken is done when internal temperature reaches 165˚F.
Remove chicken breasts from slow cooker and set aside on a plate.
Combine water and cornstarch in a mug or a small mixing bowl and whisk until incorporated; then, whisk the slurry into the wine sauce.
Add heavy cream to the wine sauce and whisk until combined.
Return chicken to the slow cooker; cover with a lid and cook on HIGH for 20 minutes, or until sauce has thickened.
Transfer chicken from slow cooker to plates; top with mushroom sauce and garnish with fresh parsley.