In a large mixing bowl, stir together the sourdough starter and 350ml of water until the starter is fully dissolved.
Add the flour. Mix the flour, water and starter together with a claw-like hand. Squeeze the dough in your hand to ensure the flour and water are fully combined. You want to hydrate the flour as much as possible.
Cover with a damp tea towel (or a shower cap) and leave to rest/prove for 8 hours.
Place a cast iron pot (with a lid) or dutch oven into the oven and preheat to 250°C.
Now that the bread dough has proved, add the salt and 20ml of water.
Incorporate the salt water into the dough by scooping your hand under the dough, lifting from the middle, and folding it back on itself. This is a stretch and fold method that will incorporate the salt water and also help maintain gluten.
Liberally flour the work surface before turning the dough out onto it. We are now going to pre-shape the dough.
Going around the dough, pull the edge to stretch it a little then fold it back on itself into the centre. Fold all the way around the perimeter twice (i.e. 720°). You should notice that the dough ball has become a little more taut.
Dust the dough with a little flour. Flip the dough over so that it is seam-side down and cover with a bowl. Leave to rest for 30 minutes.
Once the dough has rested, it is time for the final shaping. Cut a disc of baking parchment (at least 30x30cm wide)–you'll need this later.
Dust the top of the dough with flour and flip seam-side up.
Stretch the top of the dough out and fold in over on itself to the centre. Dust any excess flour off the bottom.
Next, alternating between left and right, take the side, stretch it a little and fold into the centre. This is called knitting or stitching the dough. You should notice a woven or knitted pattern in the seam as you work down the length of the dough.
When you reach the end, roll it up. When rolling, keep a bit of tension in the dough. You want to stretch it as you roll so that the surface is taut.
If you want a lozenge-shaped dough, the shaping is now done. If you want a round loaf, turn the dough 90° and roll again.
Flip the shaped loaf over and place on the disc of baking parchment. Score with a sharp knife or blade in whatever pattern you desire.
Next, carefully remove the pot from the oven.
Take the lid off the pot and delicately transfer the shaped loaf with the baking parchment still on into the pot. Place the lid back on the pot and place in the oven. Reduce the temperature to 225°C.
Reduce the temperature to 225°C. Bake for 20 minutes with the lid on.
After 20 minutes, remove the lid and bake for a further 25-30 minutes or until the loaf is a deep brown colour.
Cool on a wire rack fully before slicing (if you can resist!)