Start the dough the day before you plan to bake. About 4-6 hours before you plan to begin the dough, mix the levain. Mix together 50 grams of rested sourdough starter, 50 grams of bread flour and 50 grams of cool water. Let this activate until doubled in size and very bubbly, about 4-6 hours.

When the levain is bubbly and doubled in size, begin mixing the focaccia dough. Start with an autolyse: in the bowl of a stand mixer, mix together the 750 grams of bread flour and 600 grams of warm water. Cover with a damp towel and let rest for 30 minutes.

Add in the 150 grams of active levain, 2 grams of active dry yeast, 15 grams of salt, 15 grams of granulated sugar, and 75 grams of warm water.

Using the hook attachment on the mixer, mix on medium speed for ten minutes. The dough will look like a batter. Stop the mixer and scrape down the sides.

Continue to mix on medium high speed for 10-15 minutes. The dough will go from wet batter consistency to tacky and pulling away from the sides.

When you can reach in and stretch up the dough with your hands, transfer to the pan you are baking it in. Make sure the pan is at least 2.5 inches deep.

Begin the first series of 'slaps and folds.' Using a damp hand, lift the dough up completely, then 'slap' it down on itself and gently fold it toward you. Repeat this again, and as you slap and fold you'll start to see the dough gain more structure. Repeat until you have a tight mound of dough in the center of the pan. Cover with a damp towel and let rest for 30 minutes.

After 30 minutes, repeat the slaps and folds again. Cover and repeat this step two more times, for a total of four rounds of 'slaps and folds.'

After the last slap and fold, transfer the dough to the fridge and let it proof for 12 hours up to overnight.

The next day, remove the dough from the fridge and let it come to room temperature, about 2 hours.

Gently scoop the dough out of the pan and set it to the side. Coat the pan in ½ cup of olive oil. Add the dough back in, then 'stitch' it up the center. Cover and let the dough proof another hour and a half.

Preheat the oven to 550°F.

When the dough is ready, dab your fingers in a little of the olive oil pooling at the edges, then dimple the dough, pressing all the way to the bottom of the pan.

Sprinkle the top of the dough with flaky salt and bake for 18-19 minutes or until dark golden on top.

Remove from the oven, let it cool to the touch before removing it from the pan. Slice, and serve!