Slice chicken breasts in half lengthwise. Set aside. Add garlic and oil to crock cooker.
Season chicken with salt and pepper to taste on each side and lay in crock cooker. Pour chicken broth over chicken and cover. Cook on high 3 hours.
Whisk water with arrowroot until absorbed. Remove chicken from crock pot and keep warm. Stir in arrowroot water mixture into the bottom of the crock cooker and add mushrooms.
Add back the chicken and stir to coat chicken with sauce and mushrooms. Cover and cook an additional hour. Serve with a sprinkle of chopped fresh parsley.