Lay the pork fillets on a chopping board and make a cut across the meat lengthways through the centre, about 1.5 cm (3/4 inch) away from the other side, and open out. Scatter the rosemary leaves over both the inside and then the outside of the meat pieces and season generously with pepper.
Heat 2 tablespoons of the oil in a large frying pan and cook the onion over a medium heat, stirring, for 4 minutes until softened. Add the peaches and spices and cook for 1 minute.
Spoon half the peach mixture down the centre of one of the fillets and the remaining mixture down the centre of the other. Gently press the meat back together and tie with kitchen string in several places to hold the stuffing in place.
Heat the remaining tablespoon of oil in the cleaned frying pan and cook the pork over a gentle heat, turning frequently, for 20 minutes, covering the pan for the final 5–10 minutes of the cooking time, until cooked through and tender.