Measure 3/4 cup lukewarm water into a bowl, then stir in the yeast and a sprinkle of the flour. Stand for 5 minutes, until frothy.
Place the flour in a large bowl and make a well in the centre. Pour in the yeast mixture and olive oil, and stir with a wooden spoon until almost combined. Gather the dough together with your hands.
Turn the dough out onto a lightly floured surface and knead for 5 minutes, until smooth and elastic.
Place the dough in a lightly oiled bowl, cover with plastic wrap and stand in a warm place for 45 minutes, until doubled in size.
Preheat the oven to 200 C (400 F). Lightly oil a large baking tray. Punch the dough to expel the air, then knead again for a couple of minutes. Roll out the dough to a 34 x 24 cm oval or rectangle. Place on the prepared tray.
Scatter the grapes and walnuts over the surface and firmly press into the dough. Bake for 25 minutes, until golden brown. Cool slightly, then drizzle with the extra virgin olive oil and sprinkle with the rosemary.