Cook the potatoes, in their skins, in salted water for 15 minutes, or until tender. Drain and leave them to cool.
Meanwhile, heat half the butter in a large frying pan and cook the onions gently until they are evenly golden. Remove the pan from the heat and set aside.
Skin the cooled potatoes and cut them into slices about 5 mm thick. Add the remaining butter to the pan with the onions and, when it is melted, turn the heat up a little and add the potato slices. Cook, stirring to ensure that the potato slices do not stick together, for a further 10 minutes, or until the potatoes are well coloured. Season to taste, stir in the chopped parsley and serve the potatoes at once with steak or other meat.