Stir flour, fine salt and yeast together. Add the water and mix until no dry flour remains.

Cover and rest at room temperature 18 hours, until doubled and bubbly.

Coat a 9×13 pan with 2 tbsp olive oil. Fold the dough onto itself twice and place in the pan; rest 2 hours.

Preheat oven to 425°F (220°C). Dimple the dough with oiled fingers, drizzle the remaining oil, scatter rosemary and flaky salt.

Bake 22–25 minutes until deep golden.

Cool 10 minutes in the pan, then transfer to a rack.