Mix flour, yeast and salt. Then add water until you have a slightly wet dough. Add more water if needed. Tip dough onto a lightly floured surface and knead for 10 mins until smooth and elastic. Place the dough in an oiled bowl, cover and leave to rise until doubled in size.
While the dough is rising, cook onions in 1 tbsp olive oil for 5 mins until soft, then set aside.
When the dough has risen, knock it back and stretch it to fit an oiled Swiss roll tin about 25 x 35cm. Leave the dough to prove for about 20 mins.
Heat oven to 200C/fan 180C/bake. Spread the onions over the dough and scatter with the rosemary. Press your fingers into the dough to make dimples, drizzle the remaining oil over and scatter over the salt. Bake for 30 mins. Leave to cool, then serve cut or torn into squares.
