200ml/7fl oz stock (any kind, but beware of oversaltiness if using cubes)
1 tsp thyme leaves
Method
Preheat the oven to 220°C/425°F/Gas 7. Cut the potatoes into large chunks of roughly equal size, put them in a pan of well-salted water, bring to the boil and boil for 5 minutes.
Meanwhile, slash the top of the lamb approximately five times, about 5mm/¼in deep. Rub 1 tbsp oil all over the lamb and season well with salt and pepper. Run your fingers down the thyme sprigs, from base to tip, to scatter the leaves over the meat, reserving ½ tsp to add just before serving.