Feed your sourdough starter 4-8 hours prior to making this bread so that you have an active, bubbly starter to work with. See the blogpost for timeline details.
Whisk together the water and sourdough starter in a large bowl. Add the flour, salt, oil, and rosemary. Using a spatula or wet hand, mix the dough together until it forms a shaggy loaf.
350 grams filtered water, 60 grams active sourdough starter, 500 grams bread flour, 9 grams salt, 14 grams olive oil, 7 grams dried or fresh rosemary
Cover the dough with plastic wrap or a damp tea towel and let rest for 30-60 minutes.
Perform 2-3 sets of stretch and folds spaced thirty minutes apart. To complete one set of stretch and folds, grab one side of the dough and stretch it up, wiggling it slightly, then set it back on the lump of dough. Rotate the bread a quarter turn and stretch and fold the dough again. Work your way around the dough until the dough is smooth and elastic. (See photos for tips). Cover the dough and let rest for 8-10 hours or until it has almost doubled in size and has one or two air bubbles on the surface of the dough.
Using a bench scraper to pour the dough out onto a lightly floured work surface. Fold all four sides of the dough inwards, then coax it into a loose ball shape. Flip the dough over and further shape into a ball by gently spinning it towards you.
Place the dough in a banneton or a floured bowl. Cover and set in the refrigerator for 40-60 minutes.
Optional: Place dough in freezer for 20-30 minutes for an easier score.
Preheat the over to 450 degrees F. Set the bread on a piece of parchment paper that will fit into your Dutch oven. Score the dough and place it into a COLD Dutch oven. (see notes)
Bake with the lid on for 25 minutes. Remove the lid and bake for an additional 20-25 minutes or until the crust is golden brown.
Remove from oven and allow the bread to cool for one hour before slicing. Enjoy!