Start by sifting the flour into a large roomy bowl, holding the sieve up high to give it a good airing then add the salt, baking powder, cayenne pepper and chopped thyme and mix with a palette knife.
Next thinly pare off the potato skin using a potato peeler and grate the potato straight into the flour, using the coarse side of the grater.
Then add the spring onions and two-thirds of the cheese and mix everything together thoroughly.
After that, beat the egg gently with the milk and mustard, then pour the mixture into the bowl. Just bring it all together to a loose, rough dough, still using the palette knife, then bring the dough together lightly with your hands.
Next transfer it onto the baking sheet and pat it gently into a rough round, keeping it quite high. Now lightly press the rest of the cheese over the surface, dust generously with flour and add a few sprigs of thyme here and there.
Bake the bread on a shelf near the centre of the oven for 45-50 minutes or until golden brown.
Then remove it to a cooling rack and serve it still slightly warm if possible (but I have to say it's still divine a day later, warmed through in the oven).
If you want to make it in advance it can be frozen on the day of baking as soon as it has cooled then defrosted and warmed on the day.