Sirloin Steak with Spinach and Onions

Preheat broiler on high with a large ovenproof cast-iron skillet on a rack 4 inches below element. Season steak with salt and pepper; let stand 10 minutes. Meanwhile, stir together olives, sun-dried tomatoes, and 1 tablespoon each oil and vinegar.

Swirl 1 tablespoon oil into hot skillet. Add steak and broil until browned (do not flip), 6 to 7 minutes for medium-rare. Transfer to a cutting board; loosely cover to keep warm. Combine onion and 2 tablespoons oil in skillet; season. Broil, stirring a few times, until tender and golden brown in places, 10 to 12 minutes. Add 3 tablespoons vinegar; broil until mostly evaporated, about 1 minute.

Remove skillet from broiler. Toss spinach with onion in skillet, a few handfuls at a time, until just wilted; season. Slice steak and serve with spinach and onion, olive relish, and bread and butter.