Dissolve the yeast in the water in a bowl suitable for a stand mixer.
Mix the sourdough and salt into the water.
Add the flour and stir until the dough loosens from the sides of the bowl.
This takes about 10 minutes.
The dough is very firm compared to a normal sourdough bread dough - I like working with it.
Let the dough rest for at least 6 hours at room temperature under a shower cap.
Turn the dough out onto a floured surface.
Fold the dough over itself 5 times, until you have a tight loaf.
Pinch the seam together and let the bread rise in a proofing basket with the seam side up for 2 hours.
Preheat the oven along with the baking sheet to 280 degrees Celsius (536 degrees Fahrenheit) (convection).
Take the hot baking sheet out of the oven and place a piece of parchment paper on it.
Carefully turn the bread onto the hot baking sheet.
Make a deep cut along the length of the bread - it should be deeper than you think - about 2-3 cm. This helps to ensure that it rises upwards.
Before putting the bread in the oven, reduce the temperature to 250 degrees Celsius (482 degrees Fahrenheit) (convection).
Bake it for 10 minutes at that temperature and then 20 minutes at 230 degrees Celsius (446 degrees Fahrenheit) (convection).
Let the bread cool before slicing.