When you’re having a dinner party, it’s best to get as much ready in advance as possible. Because the potatoes have been par-boiled and the apple soaked in lemon juice, you can get away with making these rosti cakes a few hours in advance – prepare them to the end of point 5, then cover with foil or clingfilm and refrigerate, but only for 5 – 6 hours – don’t risk any longer or the potatoes and apples may start to discolour!