Marinate the lamb with the thyme leaves, half the rosemary and all the garlic. Cover and refrigerate at least 4 hours, preferably overnight.
Take the lamb out of the refrigerator 1 hour before cooking, to bring it to room temperature. After 30 minutes, sprinkle the lamb generously with salt and pepper.
Preheat the oven to 325 degrees F.
Heat a large saute pan over high heat for 3 minutes. Swirl in the olive oil, and wait a minute or two, until the pan is very hot, almost smoking. Place the lamb in the pan and sear
it on all sides until well browned and caramelized.
Transfer the lamb to a roasting rack
and reserve the pan. Arrange the thyme
sprigs and remaining rosemary
on the lamb and top with the butter.
Roast the lamb until an instant-read thermometer inserted into the center reads 120 degrees F. Let the lamb rest at least 10 minutes before slicing.
Arrange the greens on a plate and lay the sliced lamb over the top. Spoon some of the Lamb Sauce
and Salsa Verde over the meat.