Slice the potatoes to about the thickness of a magazine (half a cm). Cook in boiling water for 8-10 mins until tender. Meanwhile, heat an ovenproof frying pan or shallow casserole on a high heat. Dry-fry the lamb for 5 mins until browned, letting the meat release itself from the base before you turn it. Heat the grill to medium.
Tip the onion and most of the rosemary into the pan and fry for 3 mins until the onion is slightly softened and takes on some of the colour from the lamb. Stir in the gravy, then season with black pepper.
Drain the potato slices and lay over the meat, each one overlapping slightly, then grill for about 5 mins, until the potatoes are golden.