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1
Make the Dough In a saucepan, boil the potatoes in salted water until tender, about 25 minutes; drain.
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2
Rice or finely mash.
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3
Measure out 1 1/3 cups of potatoes.
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4
Make the Dough Meanwhile, in a medium bowl, combine the yeast with the warm water and 1/2 cup of the flour.
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5
Cover tightly with plastic wrap and let stand for about 25 minutes, until the mixture is frothy.
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6
Make the Dough 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.
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7
Switch to a dough hook and knead until a soft, slightly sticky dough forms, 5 minutes.
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8
Transfer the dough to a lightly oiled bowl; turn to coat.
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9
Let stand, covered, in a warm, draft-free spot until doubled in bulk, about 1 hour.
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10
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.
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11
Meanwhile, Make the Topping Lightly oil an 11-by-17-inch baking sheet.
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12
Transfer the dough to the baking sheet and press to form an 11-by-17-inch rectangle.
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13
Lightly brush the dough with olive oil; let stand, loosely covered, in a warm, draft-free spot until puffy, about 35 minutes.
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14
Meanwhile, Make the Topping Preheat the oven to 425.
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15
Using your fingers, make indentations all over the dough surface.
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16
Top with the leeks, cheese, sea salt and 1 tablespoon of the truffle oil.
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17
Meanwhile, Make the Topping Bake the focaccia on the middle rack of the oven for 20 minutes, until the cheese is bubbling and the crust is golden brown.
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18
Transfer the focaccia to a rack and brush with the remaining 1 tablespoon of truffle oil.
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19
Let stand for 30 minutes to cool slightly.
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20
Slice into squares and serve.
