Activate the sourdough starter

• Microwave the purified water for about 20 seconds until lukewarm.
• Mix the all-purpose flour and water in a jar, then stir in the refrigerated sourdough starter.
• Leave the jar by a window for 4.5 to 6 hours until the starter is bubbly and active.

Mix the dough

• In a bowl on a scale, whisk the activated starter with the water until it looks like milk.
• Add the bread flour and sea salt and mix everything really well.

Rest the dough

• Cover the bowl and let the dough rest.

First stretch and fold

• Pull the corners of the dough into the middle, in the bowl, until the dough becomes tough and hard to pull.

Rest again

• Cover the dough and leave it to rest.

Second stretch and fold and bulk proof

• Pull the dough back into the center, leaving it in a ball.
• Cover and proof in a warm spot by a window until the dough doubles in size.

Final fold and short rest

• Do one last stretch and fold, pulling everything into the center and forming a ball.
• Cover and let it rest.

Shape the loaf

• Turn the dough out onto a floured counter and stretch it into a long rectangle.
• Roll it up, then keep forming a ball by pulling the dough towards you and tucking the edges underneath.
• Add any fillings at this stage if you like.

Cold proof

• Line a floured proofing basket with a floured tea towel and place the dough smooth side down, pinching the edges and dusting with a little flour.
• Refrigerate overnight for cold proofing.

Score and bake

• Take the bread out of the refrigerator and score it to give it an ear.
• Bake in a preheated oven at 475 °F with the lid on.
• Remove the lid and bake until golden and crispy, watching closely as ovens vary.

Cool before slicing

• Let the loaf cool completely before slicing.