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1
Cook the potatoes until tender in boiling, sea-salted water.
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2
Drain, reserving 1 cup of their cooking liquid.
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3
Peel and mash them.
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4
In a large bowl, stir the yeast into the cup of reserved cooking water from the potatoes, cooled to tepid, permitting it to dissolve and rest for 15 minutes.
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5
Add the cooled mashed potatoes, the 3 3/4 cups of flour, olive oil, and sea salt, mixing the ingredients into a rough dough.
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6
Turn the mixture out onto a floured work space and knead it into a soft, resilient dough.
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7
This takes at least 8 minutes.
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8
Place the dough in a lightly oiled bowl, cover it tightly with plastic wrap, and let it rise for 1 1/2 hours or until it doubles in bulk.
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9
Preheat the oven to 425 degrees.
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10
Roll or stretch the risen dough out into a rather free-form oval shape.
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11
One does not strive here for symmetry.
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12
Cover the dough with a clean kitchen towel and permit it to rise for 1/2 hour.
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13
Lay the tomatoes and their liquids, the capers, and the olives over the dough, dusting the whole with the pecorino.
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14
Drop threads of olive oil over all and bake the focaccia on a parchment-lined tin or on a preheated baking stone for 20 to 25 minutes.
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15
Present the focaccia very warm.
