Strip the leaves from the rosemary, place them on a cutting board along with the garlic and salt, and mince them together.
Transfer to a small bowl and add 4 tablespoons olive oil, the lemon juice, and the black pepper.
Place the lamb, boned side up, on a platter large enough to hold it and brush the marinade all over both sides of the meat; cover and refrigerate for three hours or overnight.
When ready to cook, preheat your oven to 425°F. Place the lamb in a roasting pan and drizzle on the remaining 2 tablespoons of olive oil.
Roast in the center of the oven until an instant-read thermometer reaches 135°F for medium, about 30 to 35 minutes, or 145°F for medium-well, about 35 to 40 minutes.
Let the lamb rest for 10 minutes before slicing (its internal temperature will rise another 5 degrees).