Cut the zander diagonally into 8 equal-sized pieces, season with salt. Heat the oil in a non-stick frying pan and fry the fish for approx. 11/2 mins. each side. Remove, keep warm. Slice the celery into approx. 3 mm thick slices, stir fry in the same pan for approx. 3 mins. Reduce the heat, grate over the lemon zest, press out the juice and tip into the pan, add the butter, season and gently heat through. Serve the vegetables with the fish. Finely chop the parsley and sprinkle over the top.