Season the pork with salt and pepper. Heat the oil in a large frying pan over a high heat. Reduce the heat to medium, add the pork and fry for 3–5 minutes until browned on all sides, working in batches to avoid overcrowding the pan, and adding extra oil, if necessary. Use a slotted spoon to transfer the pork to the slow cooker.
Add the onion to the pan and fry, stirring, for 3–5 minutes