Preheat the oven to 200°C (400°F).
In a small bowl, combine the minced garlic, rosemary, thyme, salt, pepper, lemon zest, and 30ml of olive oil to form a thick paste.
Score the fat on the surface of the lamb in a diamond pattern and rub the herb-garlic paste thoroughly over the entire leg.
Parboil the potatoes in salted water for 8 minutes; drain and shake them in the pot to roughen the edges for increased surface area.
Place the lamb on a wire rack in a large roasting pan and arrange the potatoes around the base, tossing them with the remaining 30ml of olive oil.
Roast the lamb and potatoes at 200°C for 15 minutes to sear the exterior.
Reduce the oven temperature to 175°C (350°F) and continue roasting for approximately 60 to 75 minutes, or until the internal temperature of the lamb reaches 54°C (130°F) for medium-rare.
Remove the lamb from the oven and transfer to a carving board; tent loosely with foil and allow to rest for 20 minutes to redistribute juices.
If potatoes require further browning, increase oven heat to 220°C and roast while the lamb rests.
Carve the lamb against the grain and serve immediately with the crispy potatoes.