Heat oven to 160C/140 C fan/gas 3. To make the romesco sauce, mix the peppers, garlic, almonds, oil and vinegar together with some seasoning then set aside.
Put the remaining ingredients except the potatoes in a large baking dish, transfer to the oven and cook for 1 1/2 hours.
Increase the oven temperature to 180C/ 160C fan/gas 5. Remove the tray from the oven and toss in the potatoes then return the tray to the oven and cook for a further 45 minutes. If your potatoes are not quite tender, remove the lamb to rest and continue cooking your potatoes.
Serve the lamb with the potatoes, the sticky pan juices and the romesco sauce.
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