8 hours before you plan to make the dough, feed your sourdough starter.
In a medium-sized mixing bowl, add 1/2 cup bubbly sourdough starter and 1 cup water. Mix until combined.
Pour in 2 1/2 cups bread flour. Mix and head knead until a sticky and shaggy dough forms. Cover the dough with plastic wrap and let it rest for 1 hour.
After 1 hour, sprinkle 1/2 tablespoon of fine sea salt over the dough. Knead the bread just until the salt is incorporated.
Cover with plastic wrap and let rest for 1 hour.
After 1 hour, pick up the dough and fold it 6 times. Cover with plastic wrap and let rest 1 hour.
After the 3rd hour of rest, pick up the dough, fold it 6 times, and then place it on the counter to shape it into a smooth ball. To do this, use the flat of your palm to slide the dough ball towards you, then pick it up and place it further away from you, and repeat.
Prepare a proofing basket with a cloth lining by sprinkling flour on the bottom. Place the shaped sourdough in the basket and cover it with plastic wrap. Place the whole thing in the fridge for 12 hours (overnight).
The next day, when you are ready to bake, place a cast-iron Dutch oven in the oven and preheat it to 500 degrees F.
Remove the bread dough from the fridge and carefully dump it out onto a sheet of parchment paper. Score the top of your bread with a razor or sharp knife.
Once the oven is preheated, carefully remove the lid of the Dutch oven and lower the loaf into the pot using the parchment paper sides. Place the lid back on, close the oven, and bake for 20 minutes.
After 20 minutes, remove the lid to the Dutch oven, close the oven door and bake another 10 minutes.
After 30 total minutes, remove the Dutch oven and carefully, using the edges of the parchment paper, remove the loaf and place on the counter to cool.
Wait two full hours before slicing the dough so that the bread does not collapse.