Put the flours in a large mixing bowl with the salt and yeast. Whisk well to combine, then make a hole in the middle and add 100ml olive oil, plus 300ml water.
Stir together and add more water until you have a very soft, but not too sticky, dough.
Knead until soft and elastic; in a food mixer on a medium-slow speed for about 7 minutes, or by hand on a lightly oiled work surface for 10-12 minutes.
Put the dough on a lightly oiled rimless baking sheet and leave to rise in a draught-free place for 60-90 minutes until roughly doubled in size.
Knock back the dough, and lightly dust the baking tray with the semolina, if using.
Gently press the dough out with your fingers to fit the baking tray. Brush with olive oil and leave to rise again for about another hour, or until doubled in size again.
When this time has nearly elapsed, heat the oven to 220C and put a pizza stone or baking tray in there to heat up.
Whisk together the remaining 30ml olive oil with 30ml water until emulsified.
Transfer the dough to your pizza stone or baking tray and poke dimples into the surface of the focaccia with your fingertips.
Pour the oil-and-water mixture on top and scatter with the salt flakes, then place in the oven and squirt a little water into the base of the oven.
Bake for about 25 minutes, or until golden on top.
