Season the flank steak with kosher salt and black pepper.
Heat a grill or skillet over medium-high heat and add olive oil. Grill the steak for about 5 minutes on each side or until it reaches the desired doneness. Let it rest for 5 minutes and then slice thinly.
In a large bowl, combine mixed salad greens, cherry tomatoes, sliced avocado, and red onion.
In a small bowl, whisk together balsamic vinegar, Dijon mustard, honey, and olive oil to create the dressing.
Drizzle the dressing over the salad and toss to combine.
Top the salad with sliced steak and serve immediately.