Preheat grill for direct-heat cooking, or build a fire and burn down wood until only red coals and gray ash remain.

Score fat caps on lamb in 1/2-inch intervals with a sharp knife, cutting entirely through fat but not into flesh. Repeat in opposite direction to create a crosshatch pattern. Pound coriander and cumin seeds in a mortar and pestle just to crush (do not grind to a powder). Add cinnamon, thyme and rosemary leaves, oil, 4 teaspoons salt, and 3/4 teaspoon pepper, pounding and stirring just until combined. Rub spice mixture evenly over lamb. Let stand at least 30 minutes and up to 2 hours, or wrap in plastic and refrigerate up to 1 day, returning to room temperature 1 hour before grilling.

Preheat a large cast-iron skillet on grill, or place directly on hot coals. Place loins in skillet, fat-sides down; scatter rosemary sprigs around them. Cook, undisturbed, until some of fat is rendered and lamb is browned, 6 to 8 minutes. Flip and continue cooking until a thermometer inserted into thickest parts of meat registers 125 degrees to 130 degrees for medium-rare, 6 to 8 minutes more. Transfer to a cutting board; let stand 15 minutes before slicing and serving.