1. Heat the oven to 375°F (190°C) mark 5.
2. Trim any excess fat from the lamb. Peel the garlic, cut it into thin slivers and press them into the lamb.
3. Tie the sprig of rosemary over the lamb.
4. Peel the potatoes and cut into thick slices.
5. Peel and thinly slice the onions and mix with the potatoes.
6. Place them in a shallow heatproof dish, season well with salt and pepper and arrange the lamb on top.
7. Crumble the beef bouillon cube, blend with the hot water and pour over the meat and vegetables.
8. Cover with foil and roast in the oven for 30 minutes per lb (450g) plus an extra 30 minutes.
9. After the first hour remove the foil, baste the meat and vegetables and return them to the oven to continue cooking.
10. When the meat is cooked, untie the rosemary and place a fresh sprig on top.
11. Sprinkle the vegetables with chopped parsley and serve straight from the dish in which it was cooked.