Season the steak generously with salt and black pepper on both sides. Let it come to room temperature for about 15 minutes.
While the steak rests, make the chimichurri: In a bowl, combine parsley, cilantro, oregano, garlic, red wine vinegar, and red pepper flakes if using. Slowly whisk in the extra virgin olive oil until well combined. Season with salt to taste. Set aside.
Heat 1 tablespoon olive oil in a cast-iron skillet or heavy pan over high heat until shimmering. Place the steak in the pan and cook for 4-5 minutes per side for medium-rare, or until desired doneness. Use a meat thermometer for accuracy (130°F/54°C for medium-rare).
Transfer the steak to a cutting board and let it rest for 5 minutes. Then slice thinly against the grain.
In a large bowl, toss the mixed greens, cherry tomatoes, and red onion with a few tablespoons of the chimichurri sauce until lightly coated.
Divide the salad onto plates. Top with sliced steak and drizzle additional chimichurri over the steak. Serve immediately.
Tips
For best flavor, let the chimichurri sit for at least 15 minutes before serving.
Leftover chimichurri can be stored in the refrigerator for up to a week.
Substitute sirloin with flank steak or ribeye if desired.