Add 1 ¾ cups (420 mL) warm water to a large bowl. Sprinkle one (¼-ounce) package active dry yeast over the water. Whisk to combine.

Add 500 grams (3 ¾ cups) bread or all-purpose flour and mix to combine with a large spatula or wooden spoon until no dry bits remain and the mixture comes together in a uniform mass, about 1 minute. Scrape down the sides of the bowl, cover with plastic wrap, and let rest in a warm place for 30-45 minutes.

Combine 2 tablespoons Diamond Crystal kosher salt and 1 ½ tablespoons warm water in a small bowl and mix with your fingers or a spoon.

After 30-45 minutes of resting, add the saltwater on top of the dough. Press, squeeze, scrunch, and knead the dough with one hand until the water is evenly incorporated and the dough loosens, then comes back together. Shape the dough into a rough, sticky round. Cover and let sit in a warm place for 30-45 minutes.

Perform the first 'coil fold': Thoroughly wet your hands with water and gently scrape around the sides of the bowl to loosen the dough. Lift the dough until it is entirely free from the bowl and the ends drape down. Let the ends tuck under themselves as you lower the dough back into the bowl. Rotate the bowl 90 degrees and repeat the lifting and lowering process again. Cover the dough with plastic wrap and let rise for 30-45 minutes.

Repeat three more cycles of this: wet your hands, coil fold the dough, rotate 90 degrees, coil fold again, and let rise, covered, in a warm place for 30-45 minutes.

Preheat oven to 450 degrees F (=230 C, 210 C with fan). Add 2 ½ tablespoons olive oil to the bottom of a 10- or 11-inch cast iron skillet. Swirl to coat.

Perform one more (5th) coil fold, landing the dough into the skillet. Cover the skillet with a large overturned bowl. Let rise in a warm place until puffed and bubbly, 30-45 minutes.

Drizzle the dough with the remaining 2 tablespoons olive oil. With oiled fingers, gently but deeply dimple the dough all over.

Sprinkle with 1 teaspoon flaky salt and transfer to the oven. Bake until focaccia is deeply golden brown on top, about 22-28 minutes.

Let rest in the pan for 5 minutes. Carefully run a butter knife around the edges and release, using a large spatula if necessary to free the bread from the skillet. Transfer to a rack to rest until slightly cooled before cutting into slices and serving, with more flaky salt and olive oil for dipping.