Preheat the oven to 180C/160C Fan/Gas 4. Lay the sprouts in a single layer in one or two roasting tins. Add 2 tbsp oil, season and toss.
Roast for 20 minutes or until the edges begin to look brown and frazzled (they can go from frazzled to burnt very quickly, so take care). They won’t cook right through to the centre in this time but will finish cooking later.
Meanwhile heat ½ tbsp oil in a saute pan or wok and fry the bacon lardons until golden all over and cooked through. Lift out with a slotted spoon.
Remove all but 1½ tbsp of oil from the pan. Add the apples and onions to the pan and cook over a medium heat until golden and soft (though the apples shouldn’t be collapsing). It will take about 6 minutes.
Add the the sugar, vinegars, wine, mustard, bacon, and season. Toss well and cook until the wine has reduced by about half, then add the sprouts and cook until they are only just tender, but not floppy (all the juices should have reduced).
Toss in the butter, if using. Check the seasoning and serve.