Stir together the bread flour, yeast, salt and water in a large bowl until no dry patches remain. The dough will be quite wet and sticky, do not add more flour.
Pat the olives as dry as you can with kitchen paper. Add them to the dough along with the rosemary and stir until well dispersed.
Cover the bowl and leave to rise in a warm spot for around 1 ½ to 2 hours until the dough has risen. Place in the fridge to finish rising overnight; it can stay in there for up to 24 hours. The following day the dough should be very bubbly and should jiggle when you shake the bowl.
Scrape the dough out onto a floured surface. Use a dough scraper or spatula to fold the sides inwards to form a round ball, you will need to fold it about 6 times, moving around the edge of the ball of dough.
Flour the top of the dough (use part flour, part semolina for extra crunch) and place a large square of baking paper on top. Flip the dough over so that it is sitting on top of the paper (use your scraper, hand or a cake lifter), then use your dough scraper/spatula to roughly reshape it back into a round ball; it doesn't need to be perfect.
Dust the dough with flour and cover loosely with clingfilm or a clean tea towel. Leave to rise for 45-75 minutes until it has expanded by about half. It will flatten out as it sits, this is fine. When it has about 30 minutes left to rise, place your cast iron pan in the oven with the lid on and preheat to 240°C/220℃ fan/450°F/gas mark 9.
Slash the top of the dough with a lame or a sharp knife (optional) then lift up the baking paper and place it in the hot pan. Replace the lid (remember to use oven gloves!) and bake for 30 minutes, then remove the lid and bake the bread uncovered for a further 15 minutes until browned and crisp.
Lift out of the pan using the baking paper and place the bread on a wire rack to cool completely before slicing.