Heat oven to 220C/200C fan/gas 7. Mix the spices with a generous pinch of salt, then coat the lamb in the mix. Heat a large ovenproof frying pan. Sizzle the lamb, fat-side down, for 5 mins. Flip the lamb, then put the shallots, cut-side down, in the pan. Transfer the pan to the oven and roast for 10 mins for pink lamb, or 15 mins for medium. Remove the pan from the oven, then leave to stand for 5 mins.
Meanwhile, put the potatoes on to boil and simmer until just cooked. Drain, then gently crush with a potato masher. Add the spring onions and lemon zest, season, then keep warm until ready to serve.
Remove the lamb and shallots from the pan. Stir the capers, mint, lemon juice and a splash of olive oil (if needed) into the lamb juices to make a dressing. Slice the lamb and separate the shallots into layers. Spoon the potatoes onto warm plates, add slices of lamb and some shallots, then spoon over the dressing.