Preheat the oven to 200°C / 180°C fan / gas mark 6. Place the mushrooms on a baking tray, drizzle with 1 tbsp of olive oil and roast in the oven for 15 minutes. Remove and drain off any excess liquid.
While the mushrooms are roasting, heat the remaining tbsp of olive oil over medium heat in a frying pan, and add the smoked pancetta. Cook, stirring occasionally for about 3-5 minutes, until crispy. Remove the pancetta with a slotted spoon and set aside on some kitchen towel. Add the onion to the pan and cook, stirring occasionally for 5-7 minutes, until golden, then add the garlic and cook for a further minute. Add the rosemary, the chopped tomatoes, olives and pancetta back into the pan and season lightly. Bring to a boil, then simmer gently for about 7 minutes, until thickened.
Spoon the sauce onto the mushrooms and top each one with a torn piece of mozzarella. Bake for a further 5 minutes, until the mozzarella has melted. Serve with the salad.