Preheat the oven to 200°C, fan 180°C, gas 6. Heat the oil in a large-lidded flameproof casserole dish over a medium-high heat. Season the pork on both sides with black pepper, then add to the casserole dish and cook for 3 minutes on each side until browned. Transfer to a plate and keep warm.
Add the peaches to the casserole dish along with the onion, vinegar and 1 tablespoon of cold water. Season with freshly ground black pepper and cook, stirring, for 1 minute.
Return the pork to the casserole dish. Put the lid on and transfer to the oven. Roast for 8 minutes until the pork is cooked through with no pink remaining and the peaches are just tender.
While the pork is in the oven, cook the microwave mash to pack instructions.
Sprinkle the basil over the casserole and serve with the mash on the side.