Take the lamb out of the oven 1 hr before you want to cook it and let it come to room temperature. Heat your oven to 180°C/Fan 160°C/Gas 4.
Put a roasting tin in the oven to heat up. Peel and finely chop the garlic. Finely chop the mint leaves. Pop the garlic and mint in a bowl with 11/2 tbsp olive oil and some salt and pepper. Stir together to make a thick paste.
Slash the lamb shoulder all over with a small sharp knife. Rub the mint and garlic all over the lamb, pushing it into the slashes and also into the ends of the lamb joint.
Add the lamb to the hot roasting tin. Roast for 1 hr 15 mins for slightly pink meat, or 1 hr 30 mins for well done. Take the lamb out of the oven, transfer to a board and loosely cover with foil. Rest for 30-40 mins before carving and serving.
If you want to make a fresh mint sauce to go with the lamb, strip the leaves from a handful of mint leaves (around 30g) and mix with 4 tbsp each white wine vinegar, water and 1 tbsp caster sugar. Season and stir.