On the day or the night before, cut up 1 potato into quarters and boil it with its skin on until fork tender. Chill until ready to use, if making the night before.
Warm together the potato and milk.
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. Pour in the warmed milk and potato and mix on low for 6 minutes. Cover the bowl with plastic wrap and let sit 1 hour or until double in bulk.
Turn the mixer on to punch down the dough, then turn it out onto a flour work surface. Roll the dough into a flat rectangular sheet. Grease a sheet pan well with olive oil and transfer the dough to that pan. Continue to roll the dough into the corners of the pan to fit it exactly.
Prick the dough all over then rub the surface with more olive oil. Dot the surface with grapes in even rows. Cover with plastic wrap and let rise in a warm place until it reaches the top of the sheet pan.
Preheat oven to 375 degrees F.
Sprinkle with coarse sugar and bake for 25 to 30 minutes until golden brown. 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). Cut into rectangular pieces and serve.