Trim the lamb and pat it dry with paper towels. Place it on a cutting board.
Combine sumac, paprika, garlic powder, cumin (if using), salt, and pepper in a small bowl. Rub the spice mixture evenly all over the lamb, pressing it into the meat. Let it sit for 5 minutes.
Heat a large skillet over medium-high heat and add olive oil. Once shimmering, place the lamb in the pan. Sear for about 3-4 minutes on each side until a golden-brown crust forms and the lamb reaches an internal temperature of 135°F for medium-rare.
Reduce heat to medium and continue cooking, flipping occasionally, until the lamb feels firm but still slightly springy. Use tongs to rotate if needed, ensuring an even sear and internal temperature.
Remove the lamb from the skillet and transfer to a cutting board. Cover loosely with foil and let rest for 5 minutes to allow juices to redistribute, which will enhance tenderness and flavor.
Slice the rested lamb against the grain into thin strips. Arrange on a serving plate and serve immediately, showcasing the juicy interior and crispy exterior.