Place the whole unpeeled bulb of garlic into a pan of boiling water over a medium-low heat, then simmer for 15 to 20 minutes, or until softened.
When the time's up, remove the garlic, drain on kitchen paper and allow to cool slightly. Squeeze each clove of garlic into a mortar and pound to a rough paste with a pestle.
Heat 4 tablespoons of olive oil over a medium heat in a large saucepan and sauté the lamb for 5 to 7 minutes, or until browned on all sides.
Meanwhile, peel and roughly chop the tomatoes (if using fresh). Stir the tomatoes into the pan with the tomato purée, oregano, mint and garlic. Season generously with sea salt and black pepper.
Add enough water to just cover the stew and simmer for 1 hour 30 minutes to 2 hours, or until the lamb is tender and sauce has thickened.
During the last 15 minutes, squeeze in and stir through the lemon juice. Delicious served with chips.