Prepare the lamb: Use a whole leg of lamb, about 2 kg (4.4 lb). Ask your butcher to make a few cuts in the bone to make serving easier.
Prepare the potatoes: Peel 2 kg (4.4 lb) of potatoes and cut them into thick wedges. Arrange them in a large roasting pan with the lamb.
Prepare the garlic: Peel 5 garlic cloves and remove the green germ from the center. Add the garlic to the roasting pan.
Add the herbs: Place a few sprigs of fresh rosemary and thyme around the lamb and potatoes.
Prepare the lemon sauce: Squeeze the juice from 2–3 lemons into a bowl. Add 3 tablespoons of mustard and mix well.
Add the olive oil: Pour 30 ml (2 tablespoons) of olive oil into the lemon and mustard mixture. Whisk until combined.
Season the dish: Season the lamb and potatoes generously with salt and freshly ground black pepper.
Add the water: Pour 150 ml (approximately ⅔ cup) of water into the sauce and stir.
Pour over the lamb and potatoes: Pour the prepared sauce evenly over the meat and potatoes.
Add the lemon peel: Place a few pieces of lemon peel from 1 lemon in the roasting pan for extra aroma.
Cover with parchment paper: Soak a large sheet of parchment paper in water. Gently squeeze out the excess water and use it to cover the lamb and potatoes well.
Slow-roast: Place the roasting pan in a preheated oven at 160°C (320°F). Roast for approximately 3 hours, until the lamb is very tender and the potatoes are cooked through.
Serve: Remove the roasting pan from the oven. Serve the lamb with the potatoes and spoon some of the delicious lemony cooking juices over each portion.