Wash the lamb in cold running water and pat dry with a tea towel or kitchen paper.
Put the oil and butter in the pot, turn the heat to medium and when the butter foam begins to subside, put in the lamb, garlic and rosemary. Brown the meat all over. Check the garlic, if it is becoming very dark, put it on top of the lamb
Add the salt, pepper and wine. Let the wine simmer briskly for 15-20 seconds, turning the meat once or twice, lower the heat to cook at a very gentle simmer and cover the pot with the lid partly askew. Cook for 1 ½ to 2 hours, till the lamb begins to come off the bone. Turn every so often and if the liquid in the pot becomes insufficient, replenish it with a little water.
When done, transfer the lamb to a warm serving platter. Tip the pan to spoon off most of the fat. Add 2 tbsp water, raise the heat to high and, while the water boils away, use a wooden spoon to scrape the residues. Pour the pot juices over the lamb and serve.