Trim the leg of lamb and cut the meat into large chunks weighing 80–90 g (about 3 oz) each. Put them in a large enamelled cast-iron casserole, together with the chopped onions, peeled and flattened cloves of garlic, peeled, deseeded and diced tomatoes, the bouquet garni and the crushed peppercorns. Pour in the red wine and the olive oil and mix thoroughly with a wooden s