5 issues of BBC Good Food for £5
Thinly slice the potatoes on a mandolin, then soak in a large bowl of cold water for 1 hr. Drain well, then pat dry between two clean tea towels. Halve the onions, then thinly slice.
Heat 2 tbsp of oil in a large frying pan, then sauté the onions with 4 sprigs of thyme for around 10 mins, stirring once or twice. Stir in the vinegar and cook until evaporated, about 2 mins, stirring two or three times so the onions don’t burn. Heat oven to 190C/fan 170C/gas 5.
Arrange a quarter of the potatoes in a large ovenproof dish, reserving any larger slices for the top. Season, then sprinkle over leaves from 3 thyme sprigs and scatter over a third of the onions. Repeat the potato layers three times more and onions twice, seasoning in between, then finishing with a potato layer and more thyme sprigs.
Bring the stock to the boil, then ladle it over, making sure it seeps down the sides. Dot the top with half the butter, then bake, uncovered, for 45 mins.
While the potatoes are cooking, score the fat on the lamb rumps in a crisscross using the tip of a sharp knife. Cut the garlic bulb in half, place in a bowl with a drizzle of oil, the lamb and the remaining thyme, then leave to marinate. After the potatoes have been in for 45 mins, season both sides of each lamb rump.
Heat more oil in a large non-stick frying pan and when hot, brown the rumps, fat-side down, for 5 mins with the garlic bulb. Then, turn and brown the rumps all over for 5 mins more. Add the remaining butter to the pan, turning the rumps in the melted fat. Place the rumps on top of the potatoes, adding the garlic and drizzling with the pan juices.
Season again and return to the oven for about 35 mins. By now, the whole dish should be finished with cooked potatoes and tender lamb cooked to medium. Remove the lamb, then leave aside to rest for 10 mins, returning the potatoes to the oven to crisp up. To serve, slice each rump lengthways. Spoon the potatoes onto 6 warmed plates with slices of lamb on top.