Heat oven to 120C/fan 100C/gas ½. Put your largest lidded casserole on the hob and brown the seasoned leg of lamb on all sides - do this very thoroughly until it is a good dark brown as it will not brown during the cooking. (If you don't do it now, it will end up beige.)
If the lamb sticks, add a drizzle of oil - legs of lamb differ. Allow 10 mins on a high heat and put on the cooker fan to remove the smoke. Pour away any fat that has collected in the bottom of the pan.
Throw in the vegetables, followed by the garlic oil, wine and stock. Season and bring to the boil, then clap on the lid and put in the oven. Bake for 7 hrs, turning twice. After 5 hrs the meat will be cooked and offer no resistance to the knife.
There is no need to rest the meat when cooked in this way, but to finish the sauce, transfer the meat and vegetables to a serving dish.
Strain the sauce into a jug and pour or blot away the fat with kitchen paper. Pour the sauce into a pan then boil the liquid hard to reduce by a quarter, by which time it will be rich and flavoursome. Adjust seasoning, add the Armagnac if you wish, and serve alongside the lamb, which should be served with a spoon, a la cuillère as the French call it.