Whisk the flour, yeast, sugar and salt together in a large bowl. Pour in the warm water and 60 ml of olive oil and bring it together with your hands until there is no dry flour left. The dough will be slack and sticky. That is correct.
Knead in the bowl for about 5 minutes, folding the dough over itself rather than working it on a counter. Cover and leave in a warm spot for 90 minutes, until roughly doubled.
While it rises, pull the grapes off their stems, rinse them and dry them well on a towel. Wet grapes will steam the crust. Do not remove the seeds.
Oil a 33 x 23 cm sheet pan generously. Divide the dough into two pieces, one slightly larger. Press the larger piece into the pan with oiled fingers, stretching it to the corners. Let it relax for a few minutes if it fights you.
Scatter two thirds of the grapes over the base. Sprinkle with 50 g of the sugar and half the rosemary if using. Stretch the second piece of dough over the top and pinch the edges roughly to seal.
Press the remaining grapes into the top layer so they sit half buried. Drizzle with 2 tbsp olive oil, scatter the last 30 g of sugar, the rest of the rosemary and a small pinch of flaky salt.
Rest, uncovered, for 15 minutes. This step looks skippable and is not.
Bake at 200 C (400 F) for 35 to 40 minutes, until the edges are deeply caramelized and the juice has pooled and darkened against the pan. It should look almost too dark at the corners.
Cool in the pan for 20 minutes, then slide onto a rack. Eat warm or at room temperature, cut into squares, standing up if the moment calls for it.