Place pork belly fat side up in the instant pot or pressure cooker with ½ cup water. Pressure cook on high for 45 minutes. Release the steam and let the pork cool slightly. Cut the pork belly into 1-inch chunks. Season with salt and pepper and toss in ¼ cup cornstarch, shaking off the excess.

Heat 2 tablespoon oil in a saute pan or wok over medium-high heat. Add the pork belly and cook until browned on both sides. Remove from heat.

Add the celery in the same pan and saute for 2 minutes. Add the shallots, Thai chile, garlic, and ginger and saute until softened, about 1-2 minutes. Add the pork back to the pan.

Combine soy sauce with sugar and mirin in a small bowl. Pour sauce over the ingredients in the pan, tossing to coat. Sprinkle the black pepper evenly and add the scallions. Toss to coat once more and remove from heat. Serve immediately.