Pan-Seared Steaks With Balsamic Onions

Heat 2 tablespoons oil in large skillet over medium heat until shimmering. Add onions and 1/2 teaspoon salt and cook, stirring frequently, until lightly browned, about 10 minutes. Transfer onions to bowl. Wipe out skillet with paper towels.

Add remaining 1 tablespoon oil to skillet and cook over medium-high heat until just smoking.

Meanwhile, pat steaks dry with paper towels and season with salt and pepper. Cook steaks, without moving, until browned on first side, 4 to 5 minutes. Flip steaks and continue to cook until browned on second side, 3 to 4 minutes more. Transfer steaks to plate and wrap with foil.

Discard fat in skillet, return onions to pan, and set over medium heat. Add garlic and cook until fragrant, about 30 seconds. Add 1/2 cup vinegar, rosemary, and brown sugar and cook, scraping up browned bits with wooden spoon, until thick and syrupy, about 2 minutes. Pour any accumulated steak juices into pan along with remaining 1 tablespoon vinegar. Season with salt and pepper.

Slice steaks crosswise into 1/4-inch slices and transfer to platter. Spoon onions and sauce over steak.