Make the pre-fermented dough. Mix together water, flour and yeast. Let sit for 6-12 hours, preferably overnight.
Make the schiacciata dough. In a large bowl, combine the pre-fermented dough along with 500 grams flour, 250 grams water and 6 grams salt. Mix until a shaggy dough forms, then knead with your hands until the dough is smooth and elastic but still slightly sticky, 5-10 minutes. If the dough seems very sticky and isn't coming together, add a little more flour.
Add a good drizzle of olive oil to the bowl and turn the ball of dough to coat well. Then cover with a damp kitchen towel and allow to rise for two hours, folding the dough over on itself a few times every 30 minutes for this first rise.
Drizzle a sheet pan with a generous amount of olive oil. Then stretch the risen dough out onto the sheet pan, doing your best to make it reach all the corners and sides. If you're having trouble stretching the dough (it's very elastic), let it rest for about 5 minutes between stretches.
Cover the stretched dough with plastic wrap or a damp kitchen towel and allow to rise again, 2 hours.
While the dough is rising, preheat oven to 450. Once risen, use your fingertips to dimple the dough. Then coat it in another generous amount of olive oil and sprinkle with flaky sea salt.
Bake the dough for 20-25 minutes, until it is golden brown and the bottom is nice and crusty. Allow it to cool in the pan for 10 minutes before slicing into squares and then slicing across to make sandwiches.