Preheat the oven to 160°C. Place the onions and garlic on the base of a large roasting pan, arrange the thyme, bay leaves and rosemary on top and drizzle with olive oil.
Make five small slits into the skin of the lamb to create some pockets, put a pinch of herbs and a slice of garlic into each pocket. Drizzle over some olive oil and rub it in, making sure it is coated all over. Season it with salt and pepper. Put a large frying pan, or a large roasting tray onto a medium heat and gently brown the lamb on all sides. Then place the lamb onto the bed of onions and herbs, juice the lemon over the lamb and then add the juice lemon halves into the tray.
Roast it for 2 ½ hours at 160°C or until the lamb is juicy and tender.