Stir together the starter, water, and 3 cups (360g) of the flour, stirring quickly for 1 minute.

Cover the mixture and let it rest at room temperature for 4 hours.

Move the leaven to the fridge overnight (at least 12 hours). The dough should have expanded and become more relaxed overnight.

Add the remaining 2 cups (240g) flour, salt and rosemary (if using). Thoroughly combine, kneading into a shaggy dough.

Continue kneading until dough becomes a bit smoother, then fold into a round shape.

Allow the dough to rise for hour, then punch the dough down and stretch the edges of the dough to fold over (like a tri-fold brochure) a couple times. Adding these folds will help strengthen the dough’s structure, and allow you to feel how it’s progressing over time.

Depending on the vigor of your starter and the temperature of your kitchen, this can take up to 5 hours (or even longer).

Once the dough is smooth and firm, divide into two equal sized pieces and turn them out onto a floured surface.

Shape the loaves by folding the edge and pressing into the center, continuing this motion while turning the dough ball consistently.

Once the loaves are shaped, cover with a kitchen towel or plastic and leave in a warm place to let rise until roughly doubled in size (about 4 hours).

To bake, preheat your oven to 425F and bake for 30 minutes or until outside is dark brown, but not burnt.

Transfer the loaves to a wire rack to cool completely before slicing.