To make the dough: Weigh your flour; or measure it by gently spooning it into a cup, then sweeping off any excess. In a large bowl, stir together all the dough ingredients until well combined. Use a flexible spatula to vigorously beat the dough back and forth, slapping the side of the bowl, until it’s smooth and shiny, 1 to 2 minutes.  , Cover the bowl and set it aside for 30 minutes. Perform a bowl fold: Use a bowl scraper or your wet hand to grab a section of dough from one side, lift it up, and press it down to the middle to seal. Repeat, turning the bowl 90° (a quarter turn) between each stretch, until the dough won’t elongate easily, about 6 to 8 times total.  , Cover the bowl and set aside for another 30 minutes, perform a bowl fold, then re-cover and let the dough rise overnight in the refrigerator, or at room temperature for 1 1/2 to 2 hours, until doubled in size. If your dough has been refrigerated, let it temper on the counter as you prepare the potatoes. Otherwise, prepare the potatoes towards the end of the rise time. , To prepare the potatoes: Place the potatoes in a large microwave-safe bowl or baking dish. Cover with a microwave-safe cover (like a plate), and microwave in 1-minute increments, stirring between each, until the potatoes are no longer frozen, 3 to 5 minutes total. , Spread the potatoes out onto a clean, lint-free dish towel or 2 layers of paper towels and gently pat dry to remove excess moisture. Return the potatoes to the large bowl or dish and stir in oil, salt, pepper, and onion, mixing to combine. Add the cornstarch and mix until well combined. , To assemble the focaccia: Lightly grease a 12" cast iron pan or an 11" deep-dish pizza pan (see “tips” below to use a 10" pan) with nonstick spray; this ensures that the focaccia will release cleanly. Pour 1 tablespoon oil into the pan and tilt to disperse (the oil adds flavor). Transfer about 2/3 of the potato mixture (roughly 4 cups/540g) into the pan and distribute it into an even layer, pressing some up the sides of the pan to nearly reach the top. Cover the remaining potatoes and set aside. , Gently transfer the risen dough on top of the potatoes, doing your best to center it in the pan. Cover the pan with greased plastic wrap or a reusable cover and let it rise in a warm spot for 1 1/2 to 2 hours, until very puffy; it should nearly fill the pan. , Towards the end of the rise time, preheat the oven to 425°F with racks in the bottom and upper thirds. , When ready to bake, drizzle 1 tablespoon olive oil over the dough. Sprinkle the reserved 2 cups potatoes on top in an even layer. Drizzle the remaining 1 tablespoon olive oil over the potatoes, then use greased fingers to dimple the dough, pressing all the way to the bottom, so that it fills the pan. , Bake the potato focaccia on the lower rack for 38 to 40 minutes, until the potatoes are brown. If the top isn’t as dark as you’d like, transfer the pan to the upper rack for an additional 3 to 5 minutes. , Let the potato focaccia cool for 10 minutes in the pan on a wire rack, then transfer it directly to the rack to cool. Serve slightly warm or at room temperature.  , Storage information: This potato focaccia is best enjoyed fresh. Store airtight at room temperature for 1 day. Reheat leftovers, uncovered, at 350°F for 10 to 15 minutes, until warmed through. For longer-term storage, freeze individual slices.