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In a large bowl, add the sourdough starter, water and olive oil. Mix with a fork to combine. Add the flour and salt. Continue mixing with your fork until the dough becomes stiff. Then squish everything together with your hands to incorporate all of the flour. The dough will be dry and shaggy.
Cover the bowl with plastic wrap, reusable wrap or a clean, very damp kitchen towel. Let rest (autolyse) for 30 minutes or up to 1 hour, if preferred.
After the dough has rested: return to the bowl and work the dough into a rough ball, about 15 seconds.
Note: For the dough, I've given a range in water quantity (see ingredient list above). If using 250 g of water, the texture at this stage will be quite dry and stiff. If using an increased amount of water, approximately 300-325 g, the dough will be more wet. Both textures are normal, depending on which route you take.
Cover the bowl with lightly oiled plastic wrap (alternatively, transfer the dough to a high-sided dough tub with lid and measuring marks). Let rest in a warm spot to rise, ideally 70-75 F. The dough is ready when it no longer looks dense and has almost doubled in size.
Note: The bulk rise time can take anywhere from 3-12 hours depending on the temperature of your ingredients, the potency of your sourdough starter, and your current room temperature. This will be different for everyone. For example, in the summer rise times can take anywhere between 2-4 hours @ 80 F/ 26 C whereas in the winter, the dough will take about 10-12 hours @ 68º F/ 20º C. Watch the dough and not the clock.
During bulk rise, you have the option to perform a series of 'stretch & folds' to strengthen the dough. Start 30-45 minutes into the bulk rise. Gather a portion of the dough, stretch it upwards and then fold it over itself. Rotate the bowl ¼ turn and repeat this process until you have come full circle to complete 1 set. Do this once or twice spaced about 1 hour apart. Although this step is not mandatory, it has the potential to increase the total volume and height of your bread.
Note: Doughs made with 250 g of water vs. 300-325 g of water will feel completely different. The former will be more stiff; the latter will have more elasticity. When in doubt, always give the dough extra rest time in between sets if it won't stretch with ease.
Remove the dough from the bowl onto a lightly floured surface. You do not need to 'punch down' the dough; it will gently deflate as you fold and shape it.
To shape: Starting at the top, fold the dough over toward the center. Give it a slight turn, and then fold over the next section of dough. Repeat until you have come full circle. Then flip the dough over and place it seam side down. Using your hands, gently cup the sides of the dough and rotate it, using quarter turns in a circular motion. You can also pull it towards you to even out the shape. Repeat this process until you are happy with its appearance.
Note: When shaping, the idea is for the dough to catch enough surface tension on a non-floured area in order to create a tight ball. If there is flour present, it will slide around… and drive you nuts.
Now the dough needs to rise again, but for a shorter period of time. If you've used 250g water in your dough, you can do a free form second rise in the Dutch oven (if not, see note directly below). Coat the bottom of your Dutch oven with cornmeal. Alternatively, use parchment paper to prevent sticking (this is what I do, now). Place the dough inside of the pot, cover and let rise for about 30 minutes to 1 hour. The dough ready when it is slightly puffy but not double in size. Preheat your oven to 450 F/ 232 C towards the tail end of the second rise.
Note: If your dough contains more than 250g water, omit the free from rise. Your dough will spread. You'll need to do the second rise in a cloth-lined, floured proofing basket instead. The basket will hold its shape to prevent spreading.
Right before your bread goes into the oven, make a shallow slash about 2-3 inches long (or more) in the center of the dough. Use a bread lame, razor blade, sharp paring or a small serrated steak knife. The cut should be about 1/4-inch deep.
Place the bread into the oven on the center rack (lid on) and reduce the temperature to 400° F/ 204° C. Bake for 20 minutes. Remove the lid, and continue to bake (uncovered) for an additional 40 minutes or until deep, golden brown. Keep in mind that all ovens are different; you might have to make minimal adjustments to these temperatures.
You can also take the internal temperature of your bread to double check that it is done. For sourdough, it should read about 205-210º F/ 96-98º C.
Remove the bread from the oven, and cool on a wire rack for at least an hour before slicing. Don't cut too soon or else the inside will have a damp and gummy texture.