Heat one tablespoon of the olive oil in a non-stick frying pan and fry the onion over a low heat for five minutes until softened & lightly browned. Stir regularly and add the garlic for the last minute of the cooking time.
Tip the tomatoes into the pan and add the oregano and caster sugar.
Season with salt and pepper. Place over a high heat & cook the sauce for 5–8 minutes until very thick, stirring constantly.
Remove the pan from the heat and leave the sauce to cool.
Roll out the dough on a lightly floured surface into a rectangle measuring about 44cm x 32cm (17in x 13in).
Spread the tomato mixture over the dough, leaving a 2cm border around the edge.
Lay the prosciutto in a single layer over the sauce. Tear the mozzarella into small pieces and place over the prosciutto, then scatter the basil leaves on top.
Tuck in the two short ends of the dough and roll the stromboli up firmly to enclose the filling.
Place the stromboli with the seal-side down on a large baking sheet lined with baking parchment and leave to rise for 30 minutes.
Brush the remaining oil over the dough, season with a little salt and pepper and bake the stromboli until well risen and golden brown.
Slice thickly with a serrated knife and serve with some good quality olive oil and balsamic vinegar for dipping, along with a fresh green salad.