Heat oven to 220C/200C fan/gas 7. Heat the oil in a saucepan, add the garlic, fry for 1 min until golden, then add the passata and half the basil leaves. Bring to the boil and cook until it is reduced to a thick sauce, so around 3 mins. Stir in the Parma ham and sundried tomatoes, season, then set aside to cool a little.
Roll out the pizza dough to two 12-15cm circles, then split the mixture between them, piling it onto one side. Top with the mozzarella and remaining basil.
Close the calzone, pulling the side without the filling over, then crimp around the edge, so it looks like a giant Cornish pasty. Transfer to a floured baking sheet and bake for 15 mins until golden and puffed up.