Place the onions, whole, straight into medium-hot coals and allow them to roast slowly for 20-30 minutes, or until soft. Remove from the coals and scoop out the centres, leaving a 1cm border of onion.
Chop the centres finely and combine with the crème fraiche, breadcrumbs, thyme, grated cheese and parsley and season well. Spoon the mixture into the onions, sprinkle over extra cheese and drizzle with olive oil.
Return the onions to the fire and roast for a further 5 minutes, until the tops are golden and bubbly before serving.