Preheat oven to 120ºC (250ºF). Heat the oil in a heavy-based ovenproof saucepan over medium heat. Add the onion, garlic and sage and cook for 5 minutes or until browned. Add the lentils, brandy, apple juice and stock and cover with a lid. Cook in the oven for 1 hour 45 minutes.
Heat a large non-stick frying pan over high heat. Cook the duck, skin-side down, for 3–4 minutes or until browned. Turn and cook for a further 2 minutes. Transfer the duck and pan juices to the lentil mixture, cover and cook in the oven for a further 20 minutes for medium-rare or until cooked to your liking. Stir through salt and pepper and spoon onto serving plates. Slice the duck and arrange on top with the salad leaves to serve. Serves 8.