1. Heat your oven to 160C fan-forced (170C conventional). Peel and thickly slice the onions and place in the base of a lidded roasting tray. Place the garlic and rosemary in a mortar, season well with salt and pepper and pound to a coarse paste. Add the mustard and mix to combine. Rub all over the lamb and place the meat on top of the onions. Add the stout to the base of the pan and cover with a lid or foil. Roast covered for 3 hours, then turn on the oven grill, uncover the lamb, and roast for a further 15-30 minutes until the juices in the base of the pan thicken and the meat has browned. 

2. Remove the lamb from the oven and allow to rest on a platter. With a spoon, skim the oil from the roasting tray, leaving as much of the juice as possible. Place over medium heat and add the flour to the onions in the base of the pan, stirring to thicken the juices. Add the chicken stock a little at a time, stirring continuously, until a thick gravy is produced. Serve the lamb with the gravy.

Adam's tip Lamb shoulder can render a lot of oil, which you might find difficult to skim from the roasting pan. Just pour the contents of the pan into a jug and let the oil form a thick layer at the top that can easily be skimmed off.

Also try Adam Liaw's kale, sweet potato and brown butter colcannon