Sit the lamb, skin side up, on a board. Using a sharp knife insert irregular holes through the skin. Insert the garlic into the holes.
Roast for about 20 minutes until brown. Carefully turn the lamb over and roast for a further 20-30 minutes until brown all over.
Boil the onion in water for 10 minutes, then add the potatoes and boil for 4 minutes. Drain well.
Remove the lamb from the tin. Add the onions and potatoes to the tin and mix together. Add the hot stock to the tin, then put the lamb on top, with the smooth side up. Cover tightly with foil.
Reduce the temperature of the oven to 160C/140C Fan/Gas 3 and cook for 3½-4 hours, or until the lamb is completely tender. Remove the lamb from the tin and put it onto a board. Strain off the fat from the tin into a small bowl. Increase the oven to 220C/200C Fan/Gas7 and put the potatoes and onions back into the oven to brown for 25 minutes.
Spoon 3 tablespoons of fat from the tin into a saucepan. Add the flour and stir over the heat for 30 seconds. Whisk in the hot stock, wine, redcurrant jelly, Worcestershire sauce and browning. Whisk until thickened and bubbling.
Carve the lamb and serve with gravy and potatoes and onion. Serve with green vegetables.