Combine the flour, yeast and fine salt in a large bowl. Add the water and 2 Tbsp of the olive oil, and mix with a spatula until there’s no dry flour left. It will be a wet, sticky, unpromising mass. Cover.
Over the next 2 hours, do four sets of folds, half an hour apart: wet one hand, reach under one side of the dough, pull it up and fold it over the middle, then turn the bowl a quarter turn and repeat four times. Cover again between sets.
Cover well and refrigerate for 12–24 hours.
Pour 1/4 cup of the olive oil into a 20 × 30 cm tin and spread it over the base and sides. Tip the cold dough in and gently coax it toward the corners — it won’t reach yet, and that’s fine.
Cover and leave at room temperature for 2 hours, until doubled, puffy and spread to the edges.
Heat the oven to 220°C (200°C fan).
Pour the remaining oil over the top. Oil your fingers and **dimple the dough firmly all over, pushing right down to the base of the tin.** Scatter over the flaky salt and press in rosemary sprigs if using.
Bake for 22–25 minutes, until deep golden on top and the base is crisp when you lift a corner.
Lift out of the tin onto a rack straight away — left in the tin it steams and the base softens. Eat warm.