Preheat the oven to 190 C, Fan 170, Gas 5. Grease a small baking sheet.
Pare the rind from the cheese with your sharpest knife then cut the cheese into 1 cm cubes.
Sift the flour, salt and cayenne into a big mixing bowl. Grate the potato into the flour using the coarse side of the grater.
Add the spring onions, chopped thyme and two thirds of the cheese. Use a palette knife to mix everything together thoroughly.
Beat the egg gently with the milk and mustard then pour the mixture into the bowl. Using the palette knife bring it all together into a loose rough dough.
Transfer it on to the baking sheet and pat it gently into a 15 cm rough round. Lightly press the rest of the cheese over the surface, dust with a little flour and scatter over the small sprigs of thyme.
Bake on the middle shelf for 40-45 minutes or until golden brown. Remove to a cooling rack.