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Heat oven to 140C/120C fan/gas 1. Mix the garlic, oregano and olive oil with some salt and pepper. Slash the lamb all over and rub the mixture into the meat. Place into a deep casserole dish with the onions and pour over the stock, cover with a tight-fitting lid and cook 3 hrs.
Remove the lamb from the pot, stir through the peas and broad beans. Sit the lamb back on top of the vegetables and return to the oven. Increase temperature to 180C/160C fan/gas 4 and roast, uncovered, for another 20-30 mins until the lamb is browned, adding the lettuce for the final 5 mins. Allow to rest for 20 mins, then add the lemon juice and mint to the cooking juices around the lamb. Remove the string, carve into thick slices and lay them back on top of the veg to make serving easier.