1) Peel the onions but leave the root end intact. Pour about 5cm of water into a large saucepan and set up a collapsible steamer inside. Steam the onions until easily pieced by a knife, about 30 minutes. (For more tender onions you may steam them a little longer, however they might be somewhat harder to hollow out. Either way they're delicious!) Set aside to cool.
2) Slice off about a third of the top portion of each onion. Using a paring knife, cut out the insides of each onion, leaving about four outside layers intact, to make a container for stuffing. Chop the onion insides and tops.
3) Preheat the oven to 200C/Gas 6.
4) Heat a large skillet over medium-high heat. Add the bacon and cook, stirring, until crispy and the fat has rendered, about 3 minutes. Add the chopped onion, herb sprigs, salt and sugar. Season with pepper and cook, stirring occasionally, until lightly browned, about 12 minutes. Add the wine and cook until dry, scraping the bottom of the skillet with a wooden spoon to remove any browned bits, about 2 minutes. Stir in the cream and take off the heat. Remove and discard the herb sprigs. Stir in the cheese and mustard.
5) Lightly butter a baking dish. Season the inside of the onions with salt and pepper, and spoon the stuffing into each onion cavity, mounding it slightly on top. Set the onions in the dish.
6) Heat the butter in the skillet. Add the breadcrumbs and stir until they absorb the butter. Spoon the buttered breadcrumbs on top of each onion.
7) Roast the onions until well browned and heated through, about 40 minutes.
Serve warm or at room temperature.