Squeeze any excess water out of the grated potatoes, then tip into a bowl and mix well with the egg, flour, baking powder, onion and chopped sage leaves. Season well. Heat 2 tbsp oil in a large non-stick frying pan over a medium heat, and spoon the mixture in to make 4 röstis, flattening them down with the back of a spoon into disc shapes. Cook for 5 mins each side until golden brown and crisp, then drain on kitchen paper. Can be made up to 8 hrs in advance and kept in the fridge (see tip).