Heat your oven to 220°C/Fan 200°C/Gas 7. Unwrap the lamb and put in a roasting tin. Pat the meat dry. Set aside and leave it to come up to room temperature.
Take a clove from your garlic bulb. Peel and finely chop. Slice what’s left of the garlic bulb in half to make 2 rounds. Set aside.
Place the chopped garlic in a pestle and mortar. Grate in the zest of the lemon. Add a good pinch of salt and pepper. Take half the bundle of oregano and strip the leaves from their stalks. Finely chop them. Add to the garlic and lemon. Muddle it all together with a drop of olive oil to create a fragrant rub. If you don’t have a pestle and mortar, use a small bowl and a jam jar or rolling pin to crush everything together.
Roast the lamb for 25 mins, or till it has a golden crust. Add a glass of water (or wine). Cover the tin with a double layer of foil. Lower the heat to 160°C/Fan 140°C/Gas 3. Roast for 3 hrs.
Add 2 tbsp olive oil and the juice from your remaining lemon half to the leftover oregano rub. Whisk till creamy. Taste and season. If you have any honey, you can also add a dash to the dressing if you like.
Remove the tin from the oven. Carefully unwrap the lamb (mind the steam). Transfer the meat to a chopping board. Rest for 30 mins to 1 hr before serving. Serve with the juices from the bottom of the pan and the lemony dressing. Scatter the leaves from your reserved oregano sprigs over the top.
Rub half the mix all over the lamb (keep the rest for later). Tuck the remaining oregano under the lamb, saving a few sprigs. Thinly slice half the zested lemon. Tuck the slices under your lamb. Nestle the halved garlic bulb under or next to the lamb.