Preheat oven to 230C/450F/Gas 8.
Rub the meat with softened butter and place it in a roasting tin.
Using a small, sharp, pointed knife make a series of small deep slits about 4cm (1½in) apart all over the leg of lamb, inserting a sliver of garlic and a small sprig of rosemary in each slit.
Pour a tablespoon or so of the olive oil over the meat and cook for about 30 minutes.
While the meat is cooking, place the potatoes and onions in a bowl, toss together and season.
After 30 minutes, take the meat from the oven and lift it out of the tin.
Layer the potatoes and onions into the fat in the tin and pour over the stock.
Return the roasting tin to the low shelf and place the lamb directly on the shelf immediately above - so that the juices will 'weep' onto the potatoes.
Roast for about another hour depending on how well you like your meat cooked.
Dress the salad leaves with the lemon and remaining olive oil, then season with salt and pepper.
Carve the lamb and serve with the potatoes and salad.
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