Preheat the oven to 180°C conventional oven/ 160°C fan-forced (350° Fahrenheit/Gas 4). Line a large baking tray with baking paper.
Place the breadcrumbs in a large bowl and pour the milk over to soak for a few minutes. Break up the sausage meat with your hands and add it to the bowl with milk-soaked breadcrumbs. Add the beef mince (ground meat), eggs, Parmigiano Reggiano and parsley. Season well with salt and pepper, then use your hands (or you may wish to use disposable gloves) to mix until well combined.
Place a large rectangular piece of baking paper on a clean work surface. Spoon the meat mixture onto the baking sheet. Use a metal spoon to press the meat mixture into a large rectangle shape that is approximately 1.5cm thick throughout. Arrange the ham evenly over the meat mixture, leaving a 2cm border around the edges. Top the ham with chopped mozzarella.
Use the baking paper to roll the polpettone (meatloaf) from the short end to enclose the filling. Smooth the middle joint and ends of the rolled meat mixture. Remove the baking paper and transfer the polpettone to the prepared baking tray (it is easiest to roll it onto the tray). Rub olive oil all over the polpettone. Pour over the hot vegetable stock and place 2 knobs of butter on top. Bake for 45 minutes or until the polpettone is browned and cooked through.
Allow polpettone to cool for 10 minutes before slicing to serve – buon appetito.
500g beef mince (ground meat)|300g pork and fennel sausages, skin removed|20g dried breadcrumbs|60ml milk|2 eggs|20g Parmigiano Reggiano, finely grated|2 tablespoons flat-leaf parsley, finely chopped|150g thinly sliced ham|200g mozzarella, cut into small cubes|3 tablespoons extra virgin olive oil|125ml hot vegetable stock|2 knobs butter|Sea salt, freshly ground black pepper