Heat butter in a frypan over a medium heat and fry onion until it becomes soft and translucent. Preheat oven to 200°C. Lightly grease an 18cm x 7cm springform tin and press an overlapping layer of potato slices around sides. Next, create a layer of concentric rings of overlapping discs to cover base. Spread a little onion and cheese across base, season with sea salt and ground black pepper, then dot with 4 sage leaves. Cover this layer with potato, and continue alternating potato and cheese, onion and sage layers until you reach top of the springform tin. Brush pie with butter, place on a baking tray, and loosely cover with foil. Bake for 1½ hours, then remove foil and bake for 30 minutes or until potato is crisp. Remove and allow to cool a little. Cut into generous slices and serve with a green salad.