Step 1: Pat your steak dry with paper towels. Season generously with salt and freshly ground black pepper on both sides. Let it sit at room temperature for about 15-20 minutes before cooking.
Step 2: Finely chop the fresh flat-leaf parsley, oregano, and mince the garlic cloves. Combine these in a bowl. Add the red wine vinegar, red pepper flakes (if using), and a pinch of salt and pepper to the herbs. Slowly drizzle in the extra virgin olive oil, stirring constantly until well combined. Taste and adjust seasonings as needed. Let it sit for at least 10-15 minutes for the flavors to meld.
Step 3: Heat a cast-iron skillet or heavy-bottomed pan over medium-high heat until it's smoking slightly. Add a tablespoon of high-smoke-point oil. Carefully place the seasoned steak in the hot pan.
Step 4: Sear the flank or skirt steak for 3-5 minutes per side for medium-rare, depending on thickness. Use a meat thermometer to ensure it reaches your desired doneness (130-135°F for medium-rare).
Step 5: Once cooked, transfer the steak to a cutting board and let it rest for at least 5-10 minutes. This is a crucial step to ensure a tender, more flavorful steak.
Step 6: While the steak rests, prepare your salad greens and other vegetables. In a large bowl, combine mixed greens, cherry tomatoes (halved), sliced red onion, and avocado cubes.
Step 7: Thinly slice the rested steak against the grain. This makes the steak incredibly tender and easy to eat.
Step 8: Arrange the sliced steak over the prepared salad greens. Drizzle generously with the freshly made chimichurri sauce. Toss gently to combine and serve immediately.