Preheat oven to 180°C / 356°F.
Place the leg of lamb in a large baking pan. Using a meat knife make 6-7 incisions to the leg of lamb here and there. Fill the incisions with kosher salt, ground pepper, and a clove of garlic. Use the 4 out of 6 rosemary sprigs and press them inside the incisions. Toss the remaining rosemary in the pan, as well as the remaining garlic cloves.
Drizzle the lamb with olive oil and season with kosher salt and pepper all over on top as well.
Pour the wine in the pan along with enough water to cover the bottom of the pan by ¼-inch (about 1 cm) in height.
Cover the pan with a piece of parchment paper and aluminum foil on top. Secure the edges well so no steam escapes.
Bake for 3 hours.
After 2.5 hours start to prepare the potatoes.
Half fill a cooking pot with water and season with some salt. Bring to a boil over high heat.
Add the potatoes and boil for 10 minutes. Then remove from the pot and set aside.
You may save 1 liter of the water you boiled the potatoes in, to use in the gravy later on, instead of vegetable stock or plain water. It will make the gravy more creamy.
Once the lamb has cooked for 3 hours, unwrap and add the potatoes in the pan around the meat. Drizzle them with a splash of olive oil. Season the potatoes with salt, pepper, and the dried thyme.
Raise the oven's temperature to 230°C / 446°F.
Roast the lamb and the potatoes uncovered, for about 50 minutes. Flip the potatoes at least once during that time. Once the lamb and the potatoes are nicely roasted, you may serve them just like that or proceed on making the gravy.