Preheat your grill or skillet over medium-high heat.
Season the steak with salt and pepper on both sides.
Cook the steak for about 4-5 minutes per side for medium-rare.
Remove the steak from heat and let it rest for 5 minutes.
While the steak rests, prepare the salad greens in a large bowl.
Add sliced avocado, cherry tomatoes, and red onion to the greens.
In a small bowl, whisk together olive oil and balsamic vinegar.
Drizzle the dressing over the salad and toss gently to combine.
Slice the rested steak against the grain into thin strips.
Arrange the steak slices on top of the salad and sprinkle with feta cheese.