Boil the onions in plenty of water for 15 minutes, until slightly tender. Remove from the pan and allow to cool. Remove the top 1 inch of each onion and reserve. If need be, slightly trim the stalk end of the onion so that they sit flat on a roasting tray.
Cut about a heaped tbsp out from the inside of each onion, keeping the outside intact. Finely chop along with the reserved onion top.
Pre heat the oven to 400°F.
Heat a frying pan and add a little olive oil, garlic, chopped onions and a little chopped rosemary. Fry for a couple of minutes then turn the heat down and add the cream. Remove from heat and stir in the parmesan and season to taste.
Wrap a rasher of bacon around each onion and secure with the sharpened rosemary twig or half a cocktail stick.
Place the onions on the roasting tray and spoon some of the filling inside each one. Bake for 25 minutes.
Place the onions on a warmed serving plate, warm through any remaining filling and serve on the side.