Preheat oven to 350F.
Bring a pot of water to a boil, and boil the onions until tender, about 13 minutes. Drain well and let cool slightly. Cut the top and bottom of onions off horizontally. Chop the onion insides and place into a bowl. You should have just one or two outer rings of each onion intact. Add the chopped herbs and mix well.
Soak the breadcrumbs in the milk, and place raisins in warm milk to soften. Set both of these aside.
Heat the butter in a large frying pan over medium heat, and sauté the herbs and onions for 2 to 3 minutes. Season to taste with salt.
Add the breadcrumbs and raisins to the herb and onion mixture, along with 2 of the eggs and about 1/2 cup of the Parmigiano-Reggiano. Fill the boiled onions with the mixture, and arrange them in a baking dish lightly oiled or sprayed with cooking spray.
In a small bowl, beat the remaining egg with the remaining Parmigiano-Reggiano, adding a little milk. Pour the mixture over the onions. Put a flake of butter on top of each onion.
Bake until the surface is golden brown, approximately 20 minutes.