Place the flour, semolina, yeast, salt, honey, olive oil and rosemary in a large bowl, add the water and mix everything together to form a rough, slightly sticky dough. Turn it out onto an un-floured surface and knead for 10 minutes or so until it is smooth and elastic. Place the dough in a lightly greased bowl, cover and leave to rise until at least doubled in size, 1-1 ½ hours.
Line a baking sheet with greaseproof paper. Knock the risen dough back by giving it a brief so second knead. Divide it into 3 equal pices and roll each one out into a long sausage, about 35cm long; pinch the ends together tightly then plait them, pinch the other ends together tightly to seal and transfer the plait onto the prepared baking sheet. Cover with lightly oiled clingfilm and set aside to rise until doubled in size and the dough springs back slowly if you gently poke it with a finger, about 45 minutes.
Meanwhile, heat the oven to 190C/375F/gas mark 5. Once the dough is risen, brush it gently with a little olive oil and sprinkle with flaky sea salt. Bake for 30-45 minutes until golden and the loaf sounds hollow when tapped on the bottom, the internal temperature should reach 94C/200F. Transfer to a wire rack to cool before slicing.