Firstly, prepare the lamb. Heat oven to 160C/fan140C/gas 3. Wipe the meat all over and season well. Heat 3 tbsp of olive oil in a large heavy flameproof casserole, add the meat and fry all over on a fairly high heat for about 8 mins, turning until it is evenly well browned. Remove to a plate.
Thinly slice the onions. Add to the pan and fry for about 10 mins, until softened and tinged with brown. Add a few of the thyme sprigs and cook for a further minute or so. Season with salt and pepper.
Sit the lamb on top of the onions, then add the wine. Cover tightly. Bake for 3 hrs. You can make to this stage up to 2 days in advance, then reheat for 45 mins.
To finish off, strip the leaves from 2 thyme sprigs and chop them with the parsley. Scatter over before serving.