Preheat oven to 400 degrees F. Peel and trim the roots of the onions but leave them whole.
Cook the onions in a large pot of boiling water for 30 minutes, until tender. Use a slotted spoon to remove them to a plate.
Whisk the cream with the salt, pepper and thyme.
When the onions are cool enough to handle, slice each in half through the root. Place the onion halves, cut side down, in a small baking dish with the bay leaves. Pour the cream over the onions, sprinkle them with the Parmesan, and bake for 30 minutes, until golden.