Preheat your oven to 220°C/465°F. Start by trimming any excess fat on the leg of lamb. Leave the good bits on top - they will render off and provide delicious flavour to our sauce.
Place lamb into your preferred roasting pan. Then, using a sharp knife, cut slits into the lamb's flesh (top, bottom, sides).
Insert the garlic into the incisions you've made.
Season with salt, pepper, and oregano and rub well into the meat. Drizzle the olive oil and roast in your preheated oven for 30 minutes uncovered. (see note 3)
Once it has browned, remove it from the oven and turn it upside-down. Add the onions, rosemary, wine, lemon juice and water (or chicken stock).
Cover with parchment paper and cover again with aluminum foil. Turn the oven down to 180°C/350°F and bake for 2 hours - make sure to check it every half hour and baste it with the juices.
Remove again from the oven, and add the potatoes (if using). Cover with parchment paper and foil again, and cook for a further 1.5 - 2 hours or until the meat is fall-apart tender.
Remove the parchment paper and foil and cook uncovered for 20-30 minutes and allow the meat to brown. Remove again and transfer the slow-cooked lamb to a plate or platter, cover loosely with foil and allow to rest before carving and serving with the sauce. (see note 4)