Drain the parsnips well, then tip back into the pan and roughly mash with the lemon juice and half the butter. Then beat in all the other ingredients except the remaining butter, and season with salt. Heat the remaining butter in a non-stick frying pan about 28cm in diameter, and press the parsnip mixture into the pan. Cook until crisp underneath, then turn over with a fish slice. (Don’t worry if it breaks at this point, it will hold eventually.) Keep cooking until crisp on the other side, then slide onto a plate and flip back into the pan again. Keep on doing this until you have a crisp, puck-shaped cake. Serve on a plate or board, cut into wedges.