Mix the starter ingredients together in a bowl – you should have a fairly thick, sticky dough. Leave out for 15 minutes, just to give the yeast a kick start, then cover with cling film and put in the fridge for at least 24 hours, preferably for 48 hours.
When you are ready to make the bread, remove the starter dough from the fridge and allow to return to room temperature before using. It should be spongy with large, visible air bubbles on the surface and throughout the dough.
Transfer to a large bowl or the bowl of a stand mixer, then mix in the flour, water and salt. You will have a sticky dough to start with. Either turn out onto a floured surface and knead for 10–15 minutes, or knead with a dough hook for at least 5 minutes, until the dough is smooth and elastic.
Divide the dough into 12 pieces and roll into balls. Place on a lightly oiled baking tray and cover with a damp cloth. Leave to stand for 20 minutes.
To shape, take each ball, then press down lightly into a flat disc on a floured surface – it will have a diameter of around 10cm/4in. Make a cut from the centre of the disc, downwards, through to the edge, then prize the two sides apart, pinching in tips as you do so the dough looks like crescent shaped.
Make a further cut horizontally across the dough, just above the first cut, but this time just score without cutting right through. Place the crescents well-spaced out over two baking trays.
Cover with a damp cloth or oiled cling film and leave somewhere warm to rise. They should double in size and should spring back when pressed with a finger.
Preheat the oven to 200C/180C Fan/Gas 6. Boil a kettle of water and pour the water into a deep roasting dish. Put this in the bottom of the oven.
Dust the crescents with flour, then place in the oven. Bake for around 25 minutes, or until golden brown and hollow sounding when tapped.