Preheat the oven to gas 4, 180ºC, fan160ºC. Butter a large roasting tin, layer the potatoes and onions, season well. Pour over the cider and stock and cook for 20mins.
Meanwhile, mix the butter with the garlic and thyme, and season. Make about 8 deep cuts over the lamb, then using a teaspoon, push some of the butter mixture into the cuts. Spread the rest of the herby butter over the lamb.
Gently stir the potatoes, then place the lamb on a rack over the potatoes and roast for 2-2½ hours (check the packaging for cooking times). Leave to rest for 10mins before carving.
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