1) Position an oven rack in the lower third of the oven and preheat to 210C/Gas 7.
2) On a lightly floured work surface, roll out the pizza dough to a 30 to 35-inch diameter circle, about 1-cm thick. Sprinkle half of the mozzarella over the dough. Arrange the prosciutto over the cheese in a single layer. Sprinkle with the spinach. Top with the remaining cheese.
3) Take 1 end of the circle and roll the dough up into a thin cylinder, gently folding in the ends. Brush the dough with the olive oil and season with the salt. Place the dough, seam side down, on a baking tray lined with greaseproof paper and bake for 25 minutes until the top is golden brown. Allow the pizza to cool for 10 minutes.
Using a serrated knife, cut the pizza into 2-cm pieces and serve.