Preheat the oven to 190˚C/fan oven 170˚C/gas mark 5.
Grease a baking sheet with butter. Heat the oil in a medium, non-stick frying pan and add the leek and cook gently for about 10 minutes, until it is just starting to brown. Add the sage and allow to cool.
Put the leeks and sage in to a large mixing bowl with the cheese, flour, salt and some freshly ground black pepper. Add the beaten egg to the mixture. Now gradually add the buttermilk until you have a smooth dough. You might need to add the extra milk. Don’t worry if the dough is sticky. Transfer the dough to a floured worksurface and knead it to bring it together. Form the dough into a round approximately 23cm/9in in diameter.
Pop the dough onto the greased baking sheet and press it down to form a round. Make deep cuts across the top to make six wedges and bake the bread in the oven for about 30-35 minutes until golden brown. Transfer to a wire rack to cool.
The bread is best served warm, but it will keep for a couple of days if it doesn’t all disappear as soon as you serve it!