Take lamb out of the fridge 30 minutes before you intend on putting it in the oven. This allows the meat to relax, which adds to the tenderness of the meat.
Preheat oven to 400°F.
Place leg of lamb in a roasting tin and score the fat on the lamb. Make several deep incisions across the top and push small, 2-inch pieces of fresh rosemary into each incision with a slice of garlic. Rub a small amount of olive oil over the skin and season with sea salt and freshly ground black pepper.
Roast the lamb for 20 minutes in the hot oven, then turn the heat down to 350°F and cook for another hour or so for rare, 1 1/4 hours for medium and 1 1/2 hours for well-done.