750g waxy potatoes, such as red, peeled and cut into thick 2cm slices
570ml hot lamb or beef stock
green vegetables, to serve
For the gravy
3 tbsp lamb fat (see method)
50g plain flour
450ml hot lamb or beef stock
100ml red wine
1–2 tsp redcurrant jelly
Method
Preheat the oven to 220°C.
Sit the lamb, skin-side up, on a board. Season both sides with salt and pepper. Using a sharp knife, insert irregular holes through the skin. Insert the garlic and rosemary into the holes.
Place the lamb in a roasting tin and 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 stock to the tin, then put the lamb on top and cover tightly with foil.
Reduce the temperature of the oven to 160°C and cook for 3½–4 hours, or until the lamb is completely tender. Remove the lamb from the tin and place on a board. Strain off the fat from the tin into a small bowl in order to use in the gravy. Increase the oven to 220°C and put the potatoes and onions back into the oven to brown for 25 minutes.
To make the gravy, spoon the lamb fat into a saucepan. Add the flour and stir over the heat for 30 seconds. Whisk in the stock, wine and redcurrant jelly. Whisk until thickened and bubbling.
Carve the lamb and serve with gravy and potatoes and onion and with green vegetables.