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 4 of the eggs in a small saucepan and cook, covered completely with water, over high heat. Bring to the boil, then cook for a further 5 minutes. Carefully tip out the boiling water from the saucepan and fill with cold water to cool the eggs. Once cool, peel the eggs (leaving them whole).
Meanwhile, to make the meatloaf mixture, 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), remaining 2 eggs (raw), Parmigiano Reggiano (or Grana Padano) and sage. Season with salt, 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 24cm x 28cm in size. Arrange the mortadella slices over the meat mixture, then arrange the mozzarella slices over the mortadella, allowing 2cm from the edges. Place the peeled eggs in one long row along the in the centre of the meat mixture on top of the mozzarella layer.
Use the baking paper to roll the meatloaf from the short end to enclose the egg filling. Smooth the middle joint and ends of the rolled meat mixture. Remove the baking paper and transfer the meatloaf to the prepared baking tray (it is easiest to roll it onto the tray). Rub olive oil all over the meatloaf. Pour over the hot vegetable stock and place 2 knobs of butter on top. Bake for 40 minutes or until the meatloaf is browned and cooked through.
Allow the meatloaf to cool for 10 minutes before slicing to serve – buon appetito.