Bring a large pot of salted water to a boil. Add the whole, unpeeled onions and blanch for 8 minutes.

Transfer them immediately to a bowl of cold water and leave for 5 minutes until cool enough to handle.

Peel the onions, then cut each one in half crosswise through the equator so you have two cups per onion.

Using a small melon baller or teaspoon, scoop out the center layers of each half, leaving a shell 2-3 layers thick. Reserve all the scooped flesh. Set the shells in a 9x13 inch baking dish.

Soak the stale bread in the milk for 5 minutes, then squeeze out the excess moisture and crumble it into a bowl.

Finely chop the reserved onion flesh. Heat 2 tbsp olive oil in a 10-inch skillet over medium heat, add the chopped onion and garlic, and cook for 6-8 minutes until soft and pale golden.

Add the ground meat and cook, breaking it up with a wooden spoon, for 5 minutes until no pink remains. Pour in the white wine and let it evaporate for 1 minute. Season lightly with salt and pepper.

Transfer the meat mixture to a mixing bowl and let it cool for 5 minutes. Add the soaked bread, Parmesan, egg, parsley, nutmeg, remaining salt, and pepper. Mix with a fork until evenly combined.

Heat the oven to 200 C / 390 F.

Spoon the filling into each onion shell, pressing it in firmly so it mounds slightly above the rim. Sprinkle the extra 2 tbsp Parmesan over the tops and drizzle with 1 tbsp olive oil.

Pour 3 tbsp water or white wine into the bottom of the baking dish to prevent scorching.

Cover the dish tightly with foil and bake for 25 minutes. Remove the foil and bake for a further 15 minutes until the filling is browned on top and the onion shells are completely tender.

Remove from the oven and rest for 5 minutes before serving. Spoon any pan juices over the onions just before plating.