Heat the oven to 325°F. Pat the leg of lamb dry with paper towels and place it in a large roasting pan.
In a small bowl, mix the minced garlic, olive oil, rosemary, thyme, oregano, salt, and pepper until it forms a paste.
Rub the garlic herb paste all over the surface of the lamb, pressing it in so it sticks.
Arrange the potato chunks around the lamb in the pan and turn them once to coat lightly with any extra paste that falls off.
Roast for 2 hours, or until the lamb reaches 135°F in the thickest part for medium-rare. Stir the potatoes once halfway through cooking.
Remove the pan from the oven and let the lamb rest for 15 minutes before slicing. Serve the potatoes straight from the pan.