• Mix together the starter ingredients in a bowl and stir well until thoroughly mixed. Add it to a clean jar and loosely cover it. Leave it in a warm spot until it doubles in size.
• When the starter has almost finished rising, combine the flour and water in a large mixing bowl. Use a fork or wooden spoon to bring it together, then switch to wet hands to mix it into a shaggy dough ball with no dry bits of flour remaining. Cover the bowl and let it autolyze at room temperature until the starter is ready.
• Add the starter, lemon zest, rosemary and salt to the dough and squish it in using wet hands. Add in a few slap and folds to incorporate it better and start developing the structure.
• Over the next 3 hours, stretch and fold this dough every 30 minutes. Pull the dough up and over itself, turn the bowl a quarter turn, and repeat. Do this on all sides, and always use wet hands.
• After the folds, place the dough in a clean bowl and give it another hour or two to finish the bulk ferment. There should be definite signs of activity on the dough, like bubbles forming and the dough will jiggle if you give the bowl a shake. In a room temperature of around 24°C/75°F my dough needs about another 1-2 hours, but in colder temperatures it will take longer.
• Shape the sourdough into a batard or ball depending on what basket you’re using.

To shape a bâtard:

• Lightly flour the bench and tip out the dough.
• Pre-shape into a round and let it rest for 20 minutes.
• Flip it over so the smooth side is down. Gently stretch it into a rectangle.
• Fold the top third down and the bottom third up, like a pamphlet.
• Starting from the short end, roll it up tightly into a log.
• Pull the dough towards you on the bench to create surface tension.
• Place it seam-side up into a floured, lined banneton for the final proof.
• Cover the dough with a kitchen towel and cold-proof in the refrigerator for 8-24 hours.

Baking

• The next day, preheat the oven and a Dutch oven to 450°F/230°C for at least 30 minutes until well-heated.
• Remove the hot Dutch oven and flour the bottom well. Take the dough from the fridge and carefully flip it out of the basket and into the Dutch oven. If baking this in a large pot, it’s best to tip your dough onto a piece of parchment paper. This way, you can lower it into the pot.
• Score the dough using a razor blade or sharp knife.
• Bake covered with the lid for around 20 minutes. Remove the lid and bake uncovered for 20 minutes more, depending on your preference or your oven. Some ovens run hotter and may need less time, while others need more.
• Let the sourdough cool for at least two hours before slicing.