Place the whole garlic head in a saucepan of boiling water. Simmer for 15–20 minutes or until softened. Remove, drain on paper towel and allow to cool slightly. Squeeze each clove of garlic into a mortar and pound to a rough paste with a pestle.
Heat the oil over medium heat in a large saucepan and sauté the lamb for 5–7 minutes or until browned on all sides. Stir in the tomatoes, tomato purée, oregano, mint and garlic. Season generously with sea salt and freshly ground black pepper.
Add enough water to just cover the stew and simmer for 1½–2 hours or until the lamb is tender and sauce has thickened. During the last 15 minutes of cooking, stir in the lemon juice. Serve with chips.