Drizzle a little olive oil in the roasting tin and top with half of all the ingredients (except the lemon and cornflour), generously season the lamb and place on top, nestling amongst the potatoes. Cut the lemon in half, squeeze the juice over the meat and drop the lumps of lemon into the tin and scatter the remaining ingredients all over the top.
Cover the meat with baking parchment, pour in a cup of water (or wine) and tightly seal with foil - it must be air tight.
Bake in the oven for 3-4 hours (minimum 3 hours). Once done, remove the roasting tin from the oven - leave covered - and turn the oven up to 220FAN (240c).
When the oven is hot, cut open the foil (careful of the steam), drizzle a little olive oil over the top of the meat and put back in the oven for 10 minutes to colour then decant the meat and potatoes onto a platter dish. Cover with some of the used foil to keep warm.
Pour the juice from the roasting tray into a saucepan along with any leftovers (herbs, garlic, etc.), skim off any oil/fat and simmer in the saucepan (don't add the lemon as the gravy will be bitter). While simmering, make a slurry with the cornflour and COLD water (same amount of cornflour to water mixed together) and stir into the saucepan, keep stirring as it thickens. If you want a smooth gravy pass it through a sieve or leave them in if you don't mind it chunky!
Serve with a light green salad dressed in olive oil, vinegar and pinch oregano.
You can use lamb leg but I find shoulder works best.