Ingredients

• 1 1/2 pounds bavette (or flap) steak, cut into 4 pieces, or one 2–2 1/4-inch-thick boneless sirloin (about 2 pounds)
• 1 pound cipolline onions
• 1 tablespoon plus 2 teaspoons olive oil; plus more for drizzling
• Kosher salt
• 3 tablespoons unsalted butter
• 2 cups torn radicchio (such as Castelfranco, Chioggia, and/or Treviso)
• 2 teaspoons red wine vinegar

Instructions

1. Preheat oven to 350°F. Let steaks sit at room temperature 1 hour (this will help them cook more quickly and evenly).
2. Meanwhile, toss onions on a rimmed baking sheet with 1 Tbsp. oil; season with salt. Roast, turning halfway through, until deeply caramelized and tender, 25–30 minutes. Keep warm.
3. Heat a cast-iron or ovenproof skillet over medium-high. Rub steaks with 2 tsp. oil (you want just enough to coat them); season with salt. Cook steaks 25–30 seconds. Turn and cook another 25–30 seconds. Continue process of cooking and turning steaks until both sides are deeply browned and an instant-read thermometer inserted into the thickest part registers 130°F, 8–10 minutes.
4. Add butter to skillet and cook, tilting pan and spooning melted butter over meat, until steaks look glossy on top. Transfer to a cutting board and let rest 10 minutes before slicing against the grain.
5. While steaks are resting, toss radicchio and vinegar in a large bowl to combine. Drizzle with oil and season with salt; toss to coat.
6. Serve steak with onions and radicchio.