Preheat your grill or grill pan over medium-high heat until very hot.
Season the flank steak evenly on both sides with the sea salt, black pepper, and garlic powder.
Place the steak on the grill and cook for 4-5 minutes per side for medium-rare, or until it reaches your preferred level of doneness.
Remove the steak from the heat and transfer to a cutting board; let it rest for 5-7 minutes to ensure the juices redistribute.
While the steak rests, combine the mixed baby greens, halved cherry tomatoes, sliced cucumber, and thinly sliced red onion in a large serving bowl.
In a small glass jar or bowl, whisk together the extra virgin olive oil and balsamic vinegar until the dressing is fully emulsified.
Slice the rested flank steak against the grain into thin strips.
Drizzle the balsamic vinaigrette over the salad greens and toss gently to coat.
Top the salad with the sliced steak and the crumbled feta cheese before serving immediately.