Strip the leaves from 2 sprigs of rosemary and press them onto the partridge breast fillets. Season with freshly ground black pepper. Heat the oil in a large heavy based frying pan and fry them for 2-3 minutes each side until just cooked through.
Remove from the pan, keep warm. Melt the butter in the pan, add the apples, onions and remaining rosemary. Fry for 5-6 minutes until softened and lightly browned.
Pour in the wine and stir to scrape up any juice stuck to the bottom of the pan, boil to reduce slightly. Arrange the partridge breast fillets, apples and onions on warmed plates with the pan juices spooned over the top.