In a large mixing bowl, stir together the flour, semolina and salt. Stir in the dried yeast (or crumble in the fresh yeast, whichever you are using). Make a well in the centre of the flour mixture. Pour most of the water into the well along with the olive oil and bring the dough together with your hands or a wooden spoon. As the flour is incorporated the dough will start to take shape. Add the remaining water if the dough feels tight or hard.
Turn the dough out onto a clean surface and knead for 10 minutes or until the dough is smooth and elastic. Cut off a small piece of the dough and stretch part of it as thinly as you can. If you can see the shadow of your fingers through the dough - the light should shine through the dough like a window pane - without the dough tearing, it is ready to prove.
Knead the cut piece back into the dough and shape the dough into an even ball. Place in an oiled bowl, cover with a damp tea towel and allow to prove in a warm place for about 1-1½ hours, or until it has doubled in size (the temperature of your kitchen can affect the timing of this).
When the dough has risen, take it out of the bowl and knock the air out. Divide the dough into two equal portions, and shape each portion into a ball. Cover the dough with a damp cloth and allow to prove again for about 15 minutes.
Place a pizza stone or an upturned baking tray into the oven and preheat to its highest setting.
Dust the work surface liberally with semolina. Roll out one piece of the dough to form a circle that will fit on your pizza stone or baking tray.
Transfer the pizza base onto a plastic chopping board (or another upturned baking tray) dusted with semolina. The tray will be used to slide the pizza base directly onto the pizza stone.
Cover the surface with half of the tomato sauce and toppings of your choice.
When ready to bake, slide the pizza directly onto the pizza stone and cook for 8-10 minutes, or until the base is golden-brown and the toppings are bubbling. Repeat with the remaining dough, tomato sauce and toppings.