To prepare the filling, put the chopped onion, garlic, pine nuts, capers, olives, grated Parmigiano Reggiano, breadcrumbs, tomato purée, extra virgin olive oil, parsley and salt in a bowl and mix well. You may have to add more extra virgin olive oil, depending on the breadcrumbs and how much they absorb the liquids. You need to obtain a soft enough stuffing so that when you press it with your hands, it retains the shape you give it and doesn't crumble.