First make the glaze. Finely chop the leaves from half the rosemary. Mix with the redcurrant jelly and mustard powder, whisk to combine, and set aside.
Preheat the oven to gas 7, 220°C, fan 200°C. Pierce the lamb all over with a sharp knife, making pockets about 4cm (1½in) deep in all the fleshy parts. Put a slice of garlic and a couple of uncut rosemary leaves in each pocket. Season.
Spread the sliced onions in the base of a sturdy roasting tin, big enough to hold the lamb. Lay the lamb on top and cover loosely with foil.
Roast for 30 minutes then turn the oven down to gas 4, 180°C, fan 160°C and roast for a further 30 minutes. Take the lamb out of the oven, remove the foil (reserve this) and brush the redcurrant glaze mixture thickly over the lamb. Return to the oven for 25 minutes for rare lamb, 40 minutes for medium or 55 minutes for well done. Check periodically, and replace the foil if the meat browns too much or if the glaze looks as though it might burn.
Transfer the leg of lamb to a warmed platter (if you're making gravy, you will need the tin and its contents), tent with the foil and rest for 15-20 minutes before serving.
Tip Here's how to calculate cooking time if your lamb leg is a different weight: cook for 30 minutes at gas 7, 220°C, fan 200°C. Then roast at gas 4, 180°C, fan 160°C for 20 minutes per kg for rare lamb, 30 minutes per kg for medium or 40 minutes per kg for well done.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.