Peel the potatoes and soak in water for about 2 hours. Drain them well, grate and wring them through a clean cloth to dry well. At this stage you can put them through a food processor for a few moments but it is not necessary. Put with the onion, the flour, the lemon juice, the beaten egg, the seasoning and the baking powder, mixing together well.
Melt the butter in a pan and drop in spoonfuls of the mixture, frying on both sides until brown. This amount will make about eight latkes.