Rub the room-temperature lamb pieces all over with salt and black pepper, then coat them with a thin layer of softened butter. Let them rest like this for 15 minutes so the seasoning penetrates the meat.

Spread coarsely chopped onions, crushed garlic, and a bay leaf over the bottom of a deep roasting pan. Arrange the meat pieces in a single layer over this bed, and pour a cup of hot water into the corner of the pan.

Cover the pan tightly with two layers of aluminum foil. Bake in a preheated 150°C (300°F) oven for about 3.5 hours, until the meat falls apart easily with a fork. The foil traps the steam and lets the meat soften in its own juices.

Remove the foil and raise the oven temperature to 200°C (390°F); bake uncovered for another 20-25 minutes, until the top of the meat turns golden and lightly browned. Occasionally spoon the pan juices over the meat.

Let the meat rest for 10 minutes after taking it out of the oven so it reabsorbs its juices. Serve hot with the sauce collected at the bottom of the pan, alongside rice or roasted vegetables.