Heat your oven to 200°C/Fan180°C/Gas 6. Unwrap your lamb and pat dry with kitchen paper. Place it in a roasting tin and leave to come to room temperature.
Pull 2 cloves of garlic from the bulb. Peel and finely grate them. Zest the lemon. Finely chop the rosemary leaves. Stir together with 1 tbsp olive oil and plenty of salt and pepper.
Make a few small cuts all over the lamb leg with a sharp knife. Rub the lemon and rosemary marinade all over the lamb, pushing some into the cuts.
Cut the zested lemon into quarters. Nestle the garlic bulb and lemon wedges in the tin next to the lamb.
Roast the lamb in the oven for 40 mins. The lamb should be cooked medium after this time. If you would prefer it less pink, cook for a further 10-15 mins.
Remove the lamb from the tin and place on a plate, covered, to rest.
Pluck the garlic bulb from the tray. Squeeze the cloves from the papery skins and pop them back in the roasting tin. Set the lemon wedges aside.
Pop the tin on a medium heat on the hob. Pour in 200ml of beer, wine, stock or water. Simmer for 15 mins, scraping any lamb bits from the base with a wooden spoon. Taste and season with salt and pepper. Strain the gravy through a sieve and skim off any fat. Serve the lamb with the gravy and roast lemon wedges.