Remove lamb from the refrigerator 1 hour before starting to bring it to room temperature.
Preheat oven to 350 degrees F. Put the salt, black peppercorns, fennel seeds and cumin seeds in a spice grinder (or use a mortar and pestle) to roughly grind.
Rub the outside of the lamb with the garlic. Sprinkle half of the rub over one side of the lamb in a thick layer. Heat a cast iron skillet over medium heat until very hot. Add a splash of olive oil and place the lamb in the pan, seasoned side down. Sprinkle the remaining rub on the other side of the lamb. Saute until brown and seared, 5 to 6 minutes, then flip and repeat on the other side. Put the pan in the oven and roast until desired doneness (135 degrees F for medium). Remove the pan from the oven and let sit for 15 minutes (it should come up to 145 degrees F for medium while it rests).