Spray 4- to 5-quart slow cooker with cooking spray. Place onion in bottom of slow cooker. Top with pork. In small bowl, mix lemon juice, oregano, pepper, 1/2 teaspoon salt and the garlic. Pour and spread lemon mixture over pork. Cover; cook on Low heat setting 8 to 10 hours or High heat setting 4 to 5 hours or until pork is very tender.