Preheat oven to 100°c. Cut 8-10 holes into lamb's skin and fill with garlic cloves and sprigs of rosemary. Season with salt and black pepper.

Place lamb inside oven bag and place into a large roasting dish.

Add onion and tomato to lamb and tie bag according to instructions.

Roast for 8 hours

To serve, open oven bag at table and serve melt-in-the-mouth lamb with mash and pan juices.