Step 1

Pat steak dry and season both sides with 1/4 teaspoon each salt and pepper. Heat 1 tablespoon oil in a large skillet over medium-high heat. Add the steak and cook, flipping once, until an instant-read thermometer inserted in the thickest part registers 125°F for medium-rare, 4 to 5 minutes per side. Transfer to a cutting board, top with cheese and let rest for 5 minutes before slicing against the grain.

Step 2

Meanwhile, add onion to the pan and cook, stirring occasionally, until lightly browned, about 5 minutes.

Step 3

Whisk vinegar, Worcestershire, mustard, oregano, garlic and the remaining 1/4 teaspoon each salt and pepper in a large bowl. Slowly whisk in the remaining 2 tablespoons oil and any steak drippings. Add spinach and peppers and toss to coat. Serve the salad topped with the onions and steak.