Heat the oil in a large skillet over medium-high heat. Season the lamb with 1/2 teaspoon salt and 1/4 teaspoon pepper and cook to the desired doneness, 4 to 5 minutes per side for medium-rare. Transfer to individual plates.
Return the skillet to medium-high heat. Add the cauliflower and cook, tossing frequently, until golden, 1 to 2 minutes. Add the raisins and 1/2 cup water and cook, covered, until the cauliflower is just tender and the water has evaporated, 2 to 3 minutes.
Stir in the scallions and capers. Serve with the lamb.
Tip: To core a cauliflower, remove the leaves and place the head stem-side up. Carefully cut around the core as close as possible to the florets so the center comes out in a cone-shaped piece.