Using a sharp knife, score the fat and skin of the lamb (this will help the wine to permeate the meat) and put in a large dish. Rub the meat with the garlic, then pour over the red wine and add the rosemary. Cover and leave to marinate in the fridge for at least 3 hours, or overnight.
Preheat the oven to gas 8, 230°C, fan 210°C. Transfer the rosemary and half of the garlic to a large roasting tin and nestle the lamb on top.
Strain the marinade through a sieve, reserving 250ml (8fl oz) for the gravy. Scatter the lamb with the remaining garlic and drizzle over 2 tablespoons of oil. Season well. Put the lamb in the oven and cook for 30 minutes, then reduce the heat to gas 4, 180°C, fan 160°C and roast for a further 2 hours 30 minutes (or 30 minutes, per 450g (14½oz) for your weight of lamb shoulder joint).
To make the gravy, heat the remaining tablespoon of oil in a medium saucepan. Add the onion and cook over a high heat for 5 minutes, until softened. If the onion looks a little dry, add an extra splash of oil, then reduce the heat and stir in the flour to coat the onions.
Gradually add the reserved marinade to the gravy and stir until the flour has dissolved. Cook for 2-3 minutes, then add the stock and bring to the boil. Cook for a further 10 minutes, to reduce, then stir in the redcurrant jelly. Remove from the heat and pass through a sieve to remove the onions. Return the gravy to the pan, season and set aside.
When the lamb is cooked, remove from the oven and leave to rest for 10 minutes. The meat should be tender and melt away from the bone when sliced. Reheat the gravy and serve with the lamb.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home.
Why not try pairing this sumptuous roast lamb dish with this fruity, rich finest* Vina Mara Rioja Reserva red wine?