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Heat oven to 200C/180C fan/gas 6. Tip the potatoes and onions into a large roasting tin. Scatter over half the ras el hanout, drizzle with 2 tbsp of the oil and toss together until completely coated. Put in the oven for 40 mins to roast, turning everything over once or twice so it cooks evenly.
Meanwhile, heat 1 tbsp of oil in a frying pan and, one at a time, fry the fat side of the lamb well until dark brown, then remove from the pan. Remove the roasted veg from the oven. Season the lamb fat with the remaining ras el hanout, then drizzle or brush over the molasses. Put the lamb, fat-side up, among the potatoes and return to the oven. Cook for 20 mins for rare, 25 mins for pink and 30 mins for well done. Remove and leave to rest for 10 mins.
Meanwhile, mix the thyme with the remaining oil and set aside. To serve, either bring the roasting tin to the table, carve the racks into chops and serve the potatoes straight from the pan, or remove the racks, cut them into thick three-boned chops, then spoon some of the potatoes onto each plate and rest a thick chop against them. Drizzle everything with the lemon thyme oil to serve.