Typically made during the grape harvest in Italy, this flatbread is a little bit sweet, a little bit savory, and altogether irresistible.
In the bowl of a mixer fitted with the dough-hook attachment, mix together flour and yeast. Add water and mix on medium speed until combined. Cover bowl with plastic wrap; let rise in a warm place until bubbling and doubled in volume, about 2 hours.
Add salt; attach bowl to mixer fitted with the dough-hook attachment and mix on low speed 3 minutes. Increase speed to medium and mix 30 seconds more (dough should be loose and sticky).
Using a dough scraper, turn dough out onto a lightly floured surface. Fold bottom edge of dough up toward center of dough and gently pat to deflate. Fold top edge down toward center, slightly overlapping bottom edge; pat gently, tapping off excess flour as you work (dough will be difficult to handle, but try not to incorporate too much flour). Gently transfer dough to a large bowl, seam-side down. Cover with plastic wrap; let rise in a warm place until doubled in volume, about 1 hour.
Turn dough out onto a well-floured surface and repeat folding process. Return dough to bowl and cover with plastic wrap; let rise in a warm place until doubled in volume, 1 hour.
Preheat oven to 450 degrees (425 degrees if using a convection oven) with rack in center lined with a pizza stone, for at least 45 minutes.
Coat a 14-by-3-inch round cake pan with 1/3 cup oil; turn dough out into pan, coating with oil. With oiled fingertips, push dough out toward edges of pan. If dough starts to retract, cover with plastic and let stand 5 minutes, then continue until it reaches edges of pan. In a bowl, toss grapes with remaining 1 tablespoon oil; arrange evenly on top of dough, gently pressing to adhere. Cover with plastic wrap; let stand 45 minutes.
Liberally sprinkle dough with flaky salt and sanding sugar. Dough should be well oiled; if necessary, drizzle with up to 2 tablespoons more oil.
Bake, rotating pan halfway through, until bottom is golden brown, 25 to 30 minutes. Immediately transfer focaccia to a wire rack. Drizzle with any oil remaining in pan; sprinkle again with more flaky salt and sanding sugar. Let cool slightly. Focaccia may be served warm or at room temperature.