1. Preheat the oven to 240°C (th. 8). Let the butter soften at room temperature.
2. Fill a saucepan with water and heat it. Place the whole, unpeeled garlic cloves in the boiling water for 2 minutes, then drain them.
3. Next, place the leg of lamb in a large dish. Brush it with softened butter and drizzle it with olive oil. Season with salt, pepper, and sprinkle with bay leaves and thyme or rosemary leaves.
4. Pour 20 cl of water into the bottom of the dish and add the blanched garlic cloves.
5. Cook the leg of lamb in the oven, calculating 15 minutes per 500g. Baste it regularly with its cooking juices to make the meat more tender.
6. Before serving, let the leg of lamb rest for 10 minutes in the turned-off oven.