Preheat the oven to 170C. Put the olive oil and sage leaves in a pan. Fry until crisp and then immediately remove the sage leaves and set aside. Add the peeled and finely sliced onions to the same pan. Cook over a low to moderate heat for 20-30 minutes, stirring occasionally, until soft and caramelised. Set aside. Lay the puff pastry out on a lined baking tray and top with the caramelised onions, leaving a border all around. Top with the quartered large boiled potatoes and grated gruyère. Bake for 15 minutes. Remove from the oven when golden-brown, garnish with the crisp sage leaves and serve.