Slice the onion thinly and separate the celery into stalks and leaves. Thinly slice the stalks and roughly chop the leaves.

Place the pork, sake, sugar, salt, pepper, garlic, and chili powder into a resealable bag. Massage the seasonings into the pork, remove air, seal the bag, and refrigerate for 2 hours or overnight.

Heat salad oil in an oven-safe pot over medium heat. Sear the pork on all sides until browned, then remove. Add onion and celery to the pot, stir to coat with oil, and spread evenly. Return the pork to the pot, cover tightly, and bake at 170°C for 2 hours.