
Preheat the oven to 200C (400F). Get a roasting dish out and set aside.
Rub the lamb leg with olive oil and sprinkle salt and pepper over all sides to season. Place in the roasting dish, and make small incisions with a sharp knife all over the lamb.
Push the chopped garlic and rosemary into the incisions, ensuring they are evenly dispersed. Sprinkle any remaining garlic into the roasting dish. Add the chopped carrots, celery and onion into the dish around the lamb, and then pour over the diced tomatoes and water. Sprinkle the thyme over the dish and place the bay leaves in the tomato mixture.
Cover the roasting dish tightly with foil and place in the oven. Reduce the heat down to 170C and allow to cook for 3.5-4 hours.
When you are almost ready to serve, remove the lamb from the oven and baste it with some of the liquid from the dish. Place back in the oven uncovered and turn the grill on for 5 minutes to brown and crisp the top of the meat.
To serve, slice the lamb into slices with a knife and combine with the liquid and vegetables from the pan. Serve with roasted sweet potato and greens or a simple salad.