After several requests, here it is!
From The America's Test Kitchen Family Cookbook
Makes one 12 by 18 inch flatbread
prep time: 10 minutes
total time: 3 hours plus cooling time
For the best flavor, use good-quality extra-virgin olive oil
1 russet potato (8 oz), peeled and cut into 1-inch cubes
3 1/2 C all-purpose flour
1 1/2 t instant or rapid-rise yeast
1 1/4 t table salt
1 C water, warm
1/4 C extra-virgin olive oil, plus extra for the baking sheet
3/4 C parmesan cheese
1. Cover the potato with water in a small saucepan and simmer until completely cooked and easily pierced with a knife, 10 to 15 minutes. Drain and, when the potato is cool enough to handle, grate it on the large holes of a box grater. Reserve 1 1/3 C lightly packed potato.
2. Mix 3 1/4 C of flour, yeast, and table salt together in a standing mixer fitted with the dough hook. With the mixer on low speed, add the grated potato, water, and 2 T of the oil until the dough comes together, about 1 min. (Alternatively, stir together with a rubber spatula until the dough looks shaggy)
3. Increase the speed to medium-low and knead until the dough is smooth and elastic, about 10 min. (If, after 5 min, more flour is needed, add the remaining 1/4 C, 1 T at a time, until the dough clears the side of the bowl but sticks to the bottom.) (alternatively, knead by hand 10-15 minutes.)
4. Turn the dough out onto a clean counter and knead by hand to form a smooth, round ball, about 1 min. Place the dough in a lightly oiled bowl and wrap tightly in plastic wrap. Let rise in a warm place until doubled in size, 1 to 1 1/2 hours.
5. Press the dough into a well-oiled 12x18" rimmed baking sheet. Wrap loosely in lightly greased plastic wrap and let rise in a warm place until it has nearly doubled in size and springs back slowly when indented with a finger, 45 to 60 min.
6. Adjust an oven rack to the lower-middle position and heat the oven to 425 degrees. Press the dough with your fingertips to dimple it evenly throughout. Drizzle with remaining 2 T olive oil and sprinkle evenly with Parmesan. Bake until the focaccia bottom is golden and crisp, 20-25 min.
7. Slide the fococcia onto a wire rack and let cool slightly before cutting into squares.