Preheat the oven to 190 C, 170 Fan, Gas 5. Grease a small baking tray.
Sift the flour into a large mixing bowl. Add the spring onions, two thirds of the cheese, the cayenne pepper and salt. Using the coarse side of a grater grate in the celeriac.
Give everything a good mix. Beat the egg and milk together and, using a palette knife, gradually add it to the mixture until you have a loose rough dough.
Transfer it to the baking tray and, still keeping the rough texture, shape it into a round with your hands. Lightly press the remaining cheese over the surface.
Sprinkle with a little flour and bake on the middle shelf of the oven for 45-50 minutes, or until golden brown. Cool on a wire rack and eat as fresh as possible.