Trim pork of excess fat and season with salt and pepper. Warm oil in a large skillet over medium-high heat. Add pork and brown on all sides, 5 to 7 minutes.
Transfer pork to slow cooker. Pour off all but 1 Tbsp. fat from skillet. Add onion, garlic and ginger to skillet and cook, stirring, until softened, about 3 minutes. Add soy sauce, brown sugar and five-spice powder. Cook, stirring, until sugar has dissolved, about 1 minute. Pour over pork.
Cover and cook on low until meat is very tender when pierced with a knife, about 6 hours. Remove pork to a cutting board, tent with foil and let stand 10 minutes. Thinly slice against the grain and serve.