How to Make It
Make the dough. In a large bowl, whisk together the all-purpose flour, yeast, and fine sea salt. Add the cool water and, using your hands, mix everything together until you don’t see any flour.
Preheat the oven to 300ºF. Preheat a 10-inch cast-iron skillet on the stovetop, gradually raising the heat from low to medium. Once the pan is hot, melt the butter, then add the sliced onions and reduce the heat to medium-low. Cook for about 5 minutes, without stirring, so the onions start to shrink and become easier to move around the pan.