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.