Take the lamb out of the fridge 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. Pull 1 garlic
clove from the bulb. Peel and crush it. Finely chop the mint leaves. Finely grate the zest from the lemon. Pop the crushed garlic, mint and lemon zest in a bowl with 1 ½ 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 paste 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. Slice the garlic bulb across to the middle to create two rounds. Nestle them in the tin with the lamb. Quarter the lemon and add that to the 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 mins before carving and serving with the roast garlic cloves and a squeeze of the charred lemon quarters.