Preheat the oven to 220°C.
Mix the rosemary, oil, paprika, salt, and pepper and rub the mixture over both sides of the lamb.
Place onions and stock in a roasting tin, add a rack, and position the lamb on top.
Roast for 30 minutes at 220°C.
Reduce heat to 160°C, cover lamb with foil, and roast for 3 to 4 hours until tender.
Remove the lamb to rest and skim the fat from the roasting juices, reserving 30ml of fat.
Sieve the juices and onions into a jug and add boiling water to reach 570ml.
Heat the reserved fat in a saucepan, stir in flour until smooth, and whisk in the roasting juices.
Add redcurrant jelly, salt, pepper, and gravy browning if desired, then serve with the meat.