Recipe and Photography Courtesy of Kate Winsland and Guy Ambrosino • Serves 8 to 10
FOR THE DOUGH
1 small russet potato
2½ cups cold water
5 cups all-purpose flour
2½ teaspoons active dry yeast
1 teaspoon sugar
2 tablespoons olive oil, plus more for oiling the pan
FOR THE TOPPING
1 small sprig fresh rosemary
2 small sweet onions, thinly sliced
3 tablespoons olive oil
¾ teaspoon flaky sea salt
1. Peel the potato and cut into 1-inch cubes. Combine the potato and 2½ cups cold water in a small saucepan, cover and bring to a boil. Boil the potato until very tender when pierced with the tip of a knife, about 15 minutes.
2. Use a handheld blender to purée the potatoes and cooking liquid to a smooth slurry (alternately, run the mixture through a food mill or mash with a fork until as smooth as possible). Let cool until just warm (the water should not be boiling hot when added to the yeast).
3. Meanwhile, whisk together the flour, yeast, sugar and 1½ teaspoons salt in a large bowl.
4. Add the olive oil and the warm potato mixture and stir until the dough just comes together (it will be very soft and sticky).
5. Generously oil a large bowl and transfer the dough to it, rolling the dough around to coat it in the oil. Cover tightly with plastic wrap and set aside at room temperature to rise until doubled in size, about 1 hour.
6. Strip the rosemary leaves and coarsely chop them. Combine the rosemary, onions and olive oil in a bowl and toss together.
7. Heat the oven to 400°F. Lightly oil a half-sheet pan, then scrape the dough onto the pan. With lightly oiled fingers, stretch and pull the dough to fill the pan. Press your fingertips into the dough to create deep dimples. Scatter the onion mixture evenly over the dough, drizzling any remaining oil over everything. Sprinkle the sea salt over the topping.
8. Cover the pan with a clean kitchen towel and let rise until the dough is almost level with the sides of the pan, about 1 hour.
9. Bake the focaccia until golden brown, 35 to 40 minutes. Remove from the oven and let cool a few minutes before cutting into big squares.