Mix together the semolina flour, strong bread flour and salt. Add in the yeast and sugar and combine.
Make a well in the middle and pour in the olive oil. Mix together with a wooden spoon until a crumbly consistency.
Add the water bit by bit until a soft dough has formed. A d-scraper is a good tool to use here to combine it together. Make sure it is not too sticky. If you accidentally add too much water, add a tiny bit more strong bread flour. Cover with a tea towel and leave aside for 5 minutes.
Sprinkle the surface with a tbsp of semolina flour and knead the dough for 5 minutes until a soft dough has formed. Cut the dough in half and shape into two balls. Cover each ball of dough with a damp tea towel and leave to rise for 1 hour.
Pre-heat the oven to 150 degrees fan/170 degrees/gas mark 2. Dust the surface with a little more semolina flour and roll the risen dough into a flat circle of approx. 8 inch diameter. Sprinkle with nigella seeds (optional).
Grease the cast-iron pan with a little olive oil and heat on high. Cook each circle of dough for 1-2 minutes each side, until brown. Finish off both circles in the oven on a baking tray until crisp (about 10 minutes).