© Lucy Schaeffer
- 2 large baking potatoes, peeled
- 2 teaspoons active dry yeast
- 1 cup warm water
- 3 1/2 cups all-purpose flour
- 3 tablespoons extra-virgin olive oil, plus more for brushing
- 6 large leeks, halved lengthwise and sliced crosswise 1/8 inch thick
- 1 1/2 teaspoons thyme leaves
- 1/4 teaspoon salt
- 1/4 teaspoon sugar
- 2 tablespoons extra-virgin olive oil
- 1/4 pound Emmental cheese, diced
- Coarse sea salt
- 2 tablespoons white truffle oil
- MAKE THE DOUGH In a saucepan, boil the potatoes in salted water until tender, about 25 minutes; drain. Rice or finely mash. Measure out 1 1/3 cups of potatoes.
- Meanwhile, in a medium bowl, combine the yeast with the warm water and 1/2 cup of the flour. Cover tightly with plastic wrap and let stand for about 25 minutes, until the mixture is frothy.
- In the bowl of a standing electric mixer fitted with the paddle, beat the yeast mixture with the mashed potatoes, 3 tablespoons of the olive oil, 2 teaspoons of salt and the remaining 3 cups of flour at medium speed for 2 minutes. Switch to a dough hook and knead until a soft, slightly sticky dough forms, 5 minutes. Transfer the dough to a lightly oiled bowl; turn to coat. Let stand, covered, in a warm, draft-free spot until doubled in bulk, about 1 hour.
- MEANWHILE, MAKE THE TOPPING In a skillet, cook the leeks with the thyme, salt and sugar in the olive oil over moderate heat, stirring, until caramelized, 15 minutes.
- Lightly oil an 11-by-17-inch baking sheet. Transfer the dough to the baking sheet and press to form an 11-by-17-inch rectangle. Lightly brush the dough with olive oil; let stand, loosely covered, in a warm, draft-free spot until puffy, about 35 minutes.
- Preheat the oven to 425°. Using your fingers, make indentations all over the dough surface. Top with the leeks, cheese, sea salt and 1 tablespoon of the truffle oil.
- Bake the focaccia on the middle rack of the oven for 20 minutes, until the cheese is bubbling and the crust is golden brown. Transfer the focaccia to a rack and brush with the remaining 1 tablespoon of truffle oil. Let stand for 30 minutes to cool slightly. Slice into squares and serve.