Roast in the oven for 1 hour, or until the potatoes are a rich, golden colour and are crispy on the outside. Remove the tray from the oven and using a potato masher, gently squash each potato so they crack open and spread out in the tray. Pick the sage leaves from their stems and toss them in the olive oil. Sprinkle them over the potatoes, tipping in any excess olive oil, and return the tray to the oven for a further 30 minutes, or until the potatoes are completely golden. Serve immediately.