Preheat oven to 400°F.

Place bread in large bowl. Toss with milk and let sit until milk is absorbed and bread is soft, about 10 minutes.

Cut off the top ½ in (1¼ cm) of each onion, then trim the bottom so that the onion sits flat. Leave the onions in their peels. Using a spoon, scoop out centre of each onion, being careful not to pierce bottom and leaving 2 to 3 outer layers intact. Season generously with salt and pepper. Finely chop scooped-out onion interiors.

Using your hands, squeeze out any excess milk from soaked bread and drain milk. In same bowl, add sausage, parsley, Toma cheese, chopped onion and bread crumbs. Season with 1¼ teaspoons of salt and some pepper. Mix vigorously until well combined and smooth.

Place onions hollow-side up in 9×13-in (23×33-cm) baking dish. Stuff sausage mixture into onions, packing it tightly and mounding any extra filling on top. Drizzle with olive oil and sprinkle with Parmigiano Reggiano.

Place rosemary springs in baking dish and pour in ½ cup water. Cover with foil and bake for 30 minutes. Remove foil and bake uncovered for another 10 to 15 minutes, until onions are golden and filling is bubbling. Alternatively, if entertaining, you can also do this in individual baking trays (see photo). Let onions cool for 5 minutes before serving.