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On a clean kitchen towel or paper towels, spread out the frozen hash browns to defrost while you make the dough.
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Spray a half-sheet pan (18" x 13") or 9" x 13" pan with nonstick spray, then add 2 tablespoons (25g) of the oil and spread it evenly across the bottom of the pan.
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In a large mixing bowl, stir together the mix and yeast. With a flexible spatula, stir in the hot water and 2 tablespoons (25g) of the oil; mix until combined. The dough will be slack and sticky — that’s OK. Allow the dough to rest for 10 minutes, uncovered.
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Scoop the dough into the prepared pan, flip it over to coat both sides with oil, and gently spread it to the edges of the pan; the dough will be very thin and might not make it to all of the corners if using a half-sheet pan. Let the dough rise for 30 minutes, uncovered.
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At the end of the rise time, gently pat the defrosted potatoes dry with a clean kitchen towel or paper towels. In a medium mixing bowl, combine the potatoes, rosemary & sea salt packet, and the remaining 2 tablespoons (25g) oil.
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Sprinkle the potato mixture evenly across the surface of the dough. Allow it to continue rising, uncovered, for another 30 minutes.
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While the dough rises, preheat the oven to 425°F with racks in the bottom and top thirds.
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Bake the potato focaccia on the bottom rack for 30 to 40 minutes, until crisp and well-browned; if the top needs more color, transfer the focaccia to the top rack and carefully broil until golden brown, 1 to 2 minutes. Let the focaccia cool in the pan for 5 minutes then transfer it to a rack to continue cooling. Season to taste with salt and black pepper, then serve warm or at room temperature.
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Store leftover potato focaccia at room temperature, loosely covered, for up to 1 day, or refrigerate for up to 2 days. Reheat in a 350°F oven until warmed through, about 10 minutes.