Ingredients
1 1/4 pounds top sirloin steak, room temperature
Kosher salt and freshly ground pepper
1/3 cup mixed marinated pitted olives, chopped
1/4 cup chopped sun-dried tomatoes (from about 8)
1/4 cup extra-virgin olive oil, divided
1/4 cup white balsamic vinegar, divided
1 large onion, halved and thinly sliced
1 large bunch spinach (10 ounces), thick stems removed
Lightly toasted crusty bread and butter, for serving (optional)
Preparation
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.
