Fresh rosemary perfumes this moist, chewy Italian bread studded with juicy red grapes and plump raisins.
In a medium bowl, toss grapes with 1 tablespoon olive oil; set aside.
In the bowl of an electric mixer fitted with the whisk attachment, whisk together flour, water, both raisins, and yeast with a mixer on medium speed until combined. Cover with plastic wrap, and let stand in a warm place until doubled in volume and bubbling, about 2 hours.
Add salt to bowl. Switch to dough-hook attachment and mix on low speed for 5 minutes. Increase speed to medium, and mix for 30 seconds. (Dough should be loose and sticky.)
Turn dough out onto a well-floured surface using a dough scraper. Fold bottom edge of dough up toward the center and pat gently to deflate. Fold top edge down toward the center, and pat gently. (Folds should overlap slightly.) Repeat with left and right sides, tapping off excess flour as you work. (Dough will be difficult to handle, but try not to incorporate too much flour.) Gently flip dough into a large bowl. Cover with plastic wrap, and let stand in a warm place until doubled in volume, about 1 hour.
Turn dough out onto a well-floured surface, and repeat folding process. Transfer dough to bowl. Cover with plastic wrap, and let stand in a warm place until doubled in volume, about 1 hour.
Place a pizza stone on floor of a gas oven (remove racks) or bottom rack of electric oven. Preheat oven to 425 degrees for a convection oven or 450 degrees for a conventional oven for at least 45 minutes. Alternatively, if not using a pizza stone, preheat oven 15 minutes before baking.
Pour 1/3 cup oil into a 13-by-17-inch rimmed baking sheet, coating entire surface. Turn out dough onto baking sheet, coating bottom with oil. Dip fingertips in olive oil to prevent them from sticking, and push dough out toward edges of baking sheet. Cover with plastic wrap, and let stand for 45 minutes, pressing dough toward edges occasionally.
Scatter grapes over top and sprinkle with sanding sugar and rosemary. Top should be well oiled; if necessary, drizzle with up to 1 tablespoon olive oil.
Bake in lower third of oven or on preheated pizza stone, rotating halfway through, until bottom is golden brown, 30 to 40 minutes. Immediately slide focaccia out of baking sheet onto a wire rack. Drizzle with any oil left in pan; sprinkle with flaky salt. Let cool slightly. Serve warm or at room temperature.