Preheat the oven to 325°. In a large, deep ovenproof skillet, heat the oil until shimmering. Season the pork with salt and pepper and add it to the skillet in a single layer. Cook over moderately high heat, turning once, until the pork is browned, about 12 minutes. Transfer the pork to a plate. Add the onion, carrot and celery to the skillet and cook over low heat until softened, about 5 minutes.