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.
Made this tonight and everyone liked it. I added a teaspoon of chili garlic paste when sauteing the onion, garlic & ginger which gave it a great flavor. The soy flavor was a little intense, next time I'll only use half the soy and add some chicken broth. Oh, I didn't have "5 spice powder" but had a different asian blend and it seemed fine.
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