Spray 4-quart slow cooker with cooking spray; add onions to slow cooker. In small bowl, mix salt, pepper and 1/2 teaspoon of the garlic powder. Rub pork with oil, then salt mixture. Heat 10-inch nonstick skillet over medium-high heat; brown pork in skillet 2 to 3 minutes on each side. Transfer to slow cooker; cover and cook on Low heat setting 7 to 8 hours or until pork is tender.