Spray 6- to 7-quart slow cooker with cooking spray. Place onion in slow cooker.
Sprinkle both sides of pork chops with salt and pepper; place over onion. Top with mushrooms. Pour cooking sauce over mushrooms and pork chops.
Cover; cook on Low heat setting 4 to 6 hours or until pork is tender.
Remove pork chops and mushrooms to serving plate; cover to keep warm. Pour sauce from slow cooker into 3-quart saucepan; heat to boiling. Boil uncovered until reduced by half, about 20 minutes. Serve sauce over pork chops and mushrooms. Garnish with parsley.