Preheat the oven to 200C/Gas 6.
1) Place the potatoes directly on the rack in the centre of the oven and bake for 30 minutes. Pierce each potato a few times with a fork and continue to bake until tender, about 30 minutes more.
2) Remove potatoes from the oven, and turn the heat down to 190C/Gas 5.
3) Hold the potato with an oven-mit or towel, trim off the top of the potatoes to make a canoe-like shape. Reserve the tops. Carefully scoop out most of the potato into a bowl. Take care to leave enough potato in the skin so the shells stay together.
4) Mash the potato lightly with fork along with 30 to 45g of the butter and soured cream. Stir in the spring onion, nutmeg, and season with salt and pepper, to taste. Season the skins with salt and pepper. Refill the shells with the potato mixture mounding it slightly. Sprinkle the cheese on top of the potato filling. Brush the reserved top with the remaining butter and season with salt and pepper.
5) Set the potatoes and lids on a baking tray, and bake until heated through, about 20 minutes. Serve immediately.