Preheat the oven to 205ºC/400ºF.
Clean the potatoes, then prick them a few times with a fork to allow the humidity to escape without bursting the skin while cooking. Coat the potatoes with the oil, sprinkle with salt, then bake them in the middle of the oven for about 1 hour, depending on the size of the potatoes. (The baking time could be slightly reduced by wrapping each potato individually in aluminum foil, since the heat trapped inside would help cooking. The result would be a softer skin and a moist flesh).
Outdoor grill method
Clean the potatoes then wrap them individually in aluminum foil. They are ready after 45 to 60 min, depending on their size.
Clean the potatoes, then prick them a few times with a fork and place them on a paper towel in the microwave. The cooking time at maximum power is about 8 min for one potato, depending on the size of the potato and the power of the oven (The cooking time must be increased when more than one potato is cooked at the same time). Turn the potatoes once at the halfway point if the oven does not have a turntable. Let the potatoes stand 2 min, wrapped in paper towels before serving.
Halve the potatoes lengthwise, scoop out the flesh using a spoon, paying attention not to damage the "shells". Mash the flesh in a bowl, stir in the butter and chopped scallions. Season with salt and pepper, then put the mixture back into the skins. Place the potatoes back into the oven under the top broiler for 2-3 min until they are warm and slightly golden-coloured.
Alternatively, the broiling step can be skipped and the halved potatoes may be just garnished and served right away.