To make the dough, place the prepared levain starter, water, flour and salt in a large mixing bowl. Using a wooden spoon, mix all the ingredients together to form a wet but firm dough. Set aside for 20-30 minutes. This will let the flour absorb the water, making it easier to knead.
Knead the dough on a lightly floured surface for around 10 minutes then place it in a lightly floured mixing bowl and cover with a lightly floured tea towel. Let the dough rise for 3 hours. Halfway through the rise (after 1 hour 30 minutes), give the dough a few folds.
After 3 hours, turn the dough out onto a floured countertop give it a gentle fold. Let the dough rest on the work surface for 30 minutes.
Shape the dough into a boule.
Line a mixing bowl with a well-floured tea towel. Place the dough, seam-side up into the bowl and cover with another tea towel. Let the dough rise for 1-2 hours (this is the final rise).
While the dough is proofing, preheat your oven with an empty cast iron casserole dish to 245C | Gas mark 9.
Once the dough is ready, carefully take your casserole dish out of the oven (be careful, it will be VERY hot!). Tip the dough out of the bowl onto a floured surface, then make 2 deep slashes in the top of the dough using a sharp knife, making a big X. Gently pick up the dough with your hands and drop it into the hot cast iron dish.
Cover with the lid and place in the middle of the oven for 25 minutes.
After 25 minutes, take the lid off the casserole dish and continue baking for another 15-20 minutes.
Let cool on a wire rack for at least 45 minutes before cutting.