Preheat the oven to 220ºC/425ºF/Gas mark 7 (about 200ºC if you have a fan oven)

Grease a large, shallow baking tray

In a bowl, mix the flour, cream of tartar and salt. Add the butter and rub with your fingers (or use a pastry blender) until it resembles fine crumbs

Make a well in the dry flour mixture and add the buttermilk and sourdough starter and combine using a flexible plastic dough scraper/cutter. Add the cheese and blend

Transfer the dough onto a lightly floured surface and pat it down to so it's about 4cm thick

Make into scones using a round pastry cutter

Carefully transfer to the greased tray and brush the top of each scones with a bit of beaten egg or milk

Bake until risen and golden on the top, about 12-15 minutes