Whisk together the 360 g flour, 9 g salt, 5 g sugar and 3 g instant yeast in a large bowl.
Add the 284 g warm water and 18 g olive oil and stir until thoroughly combined, with no dry patches or lumps. Cover and rest for 15 minutes.
With a wet hand, grab a section of dough from the side of the bowl, stretch it up and fold it into the centre. Continue around the bowl for about 8–12 folds, then flip the dough so the smooth side faces up. Cover and rest for 15 minutes.
Repeat the bowl fold three more times, resting the dough for 15 minutes between each set. After the fourth fold, the dough should feel noticeably stronger and smoother.
Cover and leave to rise at warm room temperature for about 1 hour, until very puffy and nearly doubled.
Grease a 23 cm square metal tin and line it with a strip of baking paper to help lift the focaccia out later. Add 13 g of the olive oil to the base of the tin.
Gently transfer the risen dough into the tin and carefully flip it once so both sides are coated in oil, handling it as little as possible so it does not deflate.
Cover and leave to rise for 1–1½ hours, until marshmallowy, jiggly and nearly filling the tin.
Towards the end of the rise, heat the oven to 245°C conventional / 225°C fan.
Lightly oil your fingertips and press them firmly down through the dough to the bottom of the tin, making deep dimples about 4 cm apart without flattening the dough.
Drizzle over the remaining 13 g olive oil and sprinkle with flaky sea salt.
Bake on a lower rack for 15–18 minutes, until deeply browned on the high points and golden in the dimples.
Lift the focaccia from the tin, remove the baking paper and turn off the oven. Return the focaccia directly to the lower oven rack for 5–7 minutes, until the sides and base are crisp and golden.
