Sift flour into a large bowl. Add yeast, salt and rosemary. Make a well in the centre and pour in water. Mix with a wooden spoon until mixture begins to clump together.
Use your hand to mix in any flour in bottom of the bowl. Knead mixture roughly into a ball. Cover bowl with plastic wrap and leave for 12 hours (ideally overnight) to prove until tripled in volume.
Use a floured hand to scrape dough from bowl. Turn out onto a floured surface and knead into a ball. Cut in half. Scatter plain flour into 2 clean tea towels. Put each piece of dough onto a tea towel, wrap up and rest in a warm place for 1-2 hours or until doubled in size.
Preheat oven to 220°C. Place a 20cm (2.4L) or 22cm (3.3L) cast-iron pot with lid into oven and preheat for 15 minutes. Lift out of oven with thick oven mits. Dust some flour into bottom of the pot.
Unwrap one of the doughs and drop into pot, smooth side down with creases from underside facing up. Cover with lid and bake for 35-40 minutes until lightly golden. Uncover and bake for an extra 10 minutes or until golden
Use oven mits to tip out bread and repeat with remaining dough (see tip). Serve.
Tip: If you have two pots, both doughs can be baked together at the same time.