In a large bowl, combine the bread flour. Add the yeast and kosher salt on opposite sides of the bowl to prevent direct contact initially.

Use your fingers to mix the dry ingredients together, then create a well in the center.

Pour in the lukewarm water and stir with a wooden spoon until a wet, shaggy dough forms.

Drizzle 30 g (2 tablespoons) of olive oil over the surface of the dough. Use your fingers to rub the oil over the dough.

Gently wiggle the dough from the edges of the bowl to coat the sides and bottom with oil.

Cover the bowl tightly with plastic wrap and refrigerate for at least 18 hours, up to 72 hours.

Spray a 9×13-inch baking pan with non-stick spray and drizzle 30 g (2 tablespoons) olive oil in the center of the pan. Set aside.

Remove the dough from the fridge. Use your fingers to pull the edges of the dough towards the center while rotating the bowl, forming the dough into a ball.

Transfer the dough to the prepared baking pan, turning it a few times to coat the entire surface in olive oil.

Leave the dough uncovered to rise in a warm, draft-free place for 3-4 hours, until it nearly fills the pan.

Towards the end of the second rise, preheat the oven to 425°F (218°C).

Drizzle the remaining 30 g (2 tablespoons) olive oil over the dough. Rub it over the surface, then sprinkle the chopped rosemary (if using) evenly.

Use your fingers to press straight down into the dough, creating deep dimples all over. Gently stretch the dough to fill the corners of the pan, if needed.

Sprinkle flaky salt over the top.

Bake for 25-30 minutes, or until golden brown and pulling away from the sides of the pan.

Let the focaccia cool in the pan for 5 minutes, then carefully transfer it to a wire rack (use a large metal spatula if needed) to prevent the bottom from getting soggy. Allow it to cool for 10 minutes before slicing and serving.