To make schiacciata, follow the instructions for making olive oil dough. Once the dough has finished its bulk prove, you can shape the schiacciata.
Divide the dough into two even-sized portions. Use a rolling pin to roll out each portion to create a 30 x 15 cm (12 x 6 inch) rectangle, about 5 mm (¼ inch) thick all over, making sure you let the dough rest between each roll for a couple of minutes. Gently transfer the dough to a baking tray lined with baking paper. Set aside in a warm place (25°C/77°F) to prove for 20 minutes.
Preheat the oven to 220°C (425°F/Gas 7). Pick and wash the grapes and place in a bowl. Crush them roughly with your fists to get some of the juice out. (Do not pound them to a paste, as you still want to see that they are grapes.) Drain the juice. Check the dough with a prod of your finger; it should rise back steadily and look puffy and glossy.
Scatter the grapes over the dough, leaving a 5 mm (¼ inch) border around the edges. Sprinkle the rosemary on top and then the sugar, if using.
Reduce the oven temperature to 200°C (400°F/Gas 6) and bake for 25 minutes, turning after 10 minutes. This bread does not need steaming as there is already a lot of moisture coming from the grapes. It’s important to check the base to see it is cooked all the way through. Schiacciata is best cooked on the hearth, so if you have a pizza stone, this is the time to use it.