Pat the flank steak dry and season both sides with kosher salt and black pepper.
Heat 2 tablespoons of olive oil in a cast-iron skillet over high heat until shimmering.
Sear the steak for 4-5 minutes per side until an internal temperature of 135°F (57°C) is reached for medium-rare.
Transfer steak to a resting board for 10 minutes to allow juices to redistribute.
In a small bowl, whisk together extra virgin olive oil, balsamic vinegar, Dijon mustard, and minced garlic until fully emulsified.
In a large mixing bowl, combine the spring greens, halved cherry tomatoes, and sliced red onion.
Thinly slice the rested steak against the grain at a 45-degree angle.
Toss the greens with half of the vinaigrette and portion onto plates.
Top each serving with steak slices and avocado, then drizzle with the remaining dressing.