Put the potatoes in a pan of cold, salted water. Bring up to the boil and cook until tender, about 8–12 minutes, depending on size. Once cooked, drain, leave in the colander for a couple of minutes to let the excess heat evaporate, then tip them back in the pan. Mash them coarsely, leaving them a little chunky.
While the potatoes are cooking, melt the butter with the oil in a large fryi