1 hour before cooking, remove Ovation butterflied lamb leg from the refrigerator to come to room temperature.
Preheat oven to 220°C fan.
Open out the lamb leg and pat dry. If there are parts of the lamb that are significantly thicker, use a sharp knife to make a horizontal cut into the thick parts and fold out flat like a book and create a more uniform shape. Cover the meat with plastic wrap and firmly bash with a rolling pin to flatten to even thickness.
Remove plastic wrap and season the lamb well with salt and pepper. Spread with pesto, followed by sun-dried tomatoes and finish by pressing feta into the lamb. Roll up the lamb tightly and tie with kitchen string at intervals along the length, finishing by securing the roast from end to end.
Lay the sliced onion in the middle of a roasting pan and place the tied roast on top with seam side down. Drizzle with olive oil and season with salt and pepper. Carefully pour the chicken stock to the onion layer.
Place into the oven and roast for 20 minutes at 220°C, then turn heat down to 180°C and roast for a further 40 minutes.
Remove from oven and rest 15 minutes. Carefully remove the kitchen string and slice. Spoon over pan juices before serving.