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1
On the day or the night before, cut up 1 potato into quarters and boil it with its skin on until fork tender.
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2
Chill until ready to use, if making the night before.
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3
Warm together the potato and milk.
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4
Meanwhile, place the flour, salt, olive oil, and yeast in a mixing bowl with a dough hook making sure the yeast isn't touching the salt.
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5
Pour in the warmed milk and potato and mix on low for 6 minutes.
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6
Cover the bowl with plastic wrap and let sit 1 hour or until double in bulk.
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7
Turn the mixer on to punch down the dough, then turn it out onto a flour work surface.
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8
Roll the dough into a flat rectangular sheet.
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9
Grease a sheet pan well with olive oil and transfer the dough to that pan.
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10
Continue to roll the dough into the corners of the pan to fit it exactly.
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11
Prick the dough all over then rub the surface with more olive oil.
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12
Dot the surface with grapes in even rows.
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13
Cover with plastic wrap and let rise in a warm place until it reaches the top of the sheet pan.
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14
Preheat oven to 375 degrees F.
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15
Sprinkle with coarse sugar and bake for 25 to 30 minutes until golden brown.
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16
Remove from oven and let cool in pan only a few minutes, then slide it out of the pan (steam gets trapped under the dough and can get soggy if you leave it in the pan too long).
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17
Cut into rectangular pieces and serve.
