Preheat the oven to 180°C (350°F/Gas 4). Trim the excess fat from the beef. Halve the sirloin lengthways, then cut each piece widthways into 3 equal pieces so you have 6 × 200 g (7 oz) portions. Heat the oil in a large ovenproof frying pan. Sear the beef until browned all over. Place the pan in the oven and roast for 3 minutes, then turn the beef over and roast for another 3 minutes for medium-rare. Rest in a warm place until required. When ready to serve, carve each piece in half diagonally.