Remove the papery skin from the onions and boil them for 10-15 minutes until they just start to become tender.
Remove from the water and allow to cool.
Preheat the oven to 400° F.
Cut the root stem off so that the onion can sit flat, and discard the root stem.
Cut the top inch or so off, and using a spoon, carve out the center of the onion to allow for a tablespoon or so of filling. Do not cut all the way to the bottom or the filling will escape during baking.
Repeat with all the onions.
Chop the removed center and tops.
Sauté the chopped onion, garlic and 1 teaspoon of chopped rosemary with the olive oil until tender.
Turn off the heat.
Add the cream and Parmesan to the onions and garlic mixture, stirring to mix well.
Season with salt and pepper to taste.
Wrap the carved onions around the middle with bacon, securing with a rosemary sprig or a toothpick.
Set the wrapped onions in an oven pan.
Divide the filling equally between the onions, filling the centers and covering the tops fully. You may have some filling left over.
Bake for 25 minutes or until the tops are nicely caramelized. You can use the broiler for a minute or two to add color, if necessary.