Preheat your grill or grill pan to medium-high heat.
Season the flank steak evenly on both sides with half of the sea salt, black pepper, and dried oregano.
Lightly brush the grill with a tiny amount of olive oil and grill the steak for 4-5 minutes per side for medium-rare, or until it reaches your desired doneness.
Remove the steak from the grill and let it rest on a cutting board for at least 5 minutes to lock in the juices.
In a small bowl, whisk together the remaining olive oil, lemon juice, and the rest of the salt, pepper, and oregano to create the vinaigrette.
In a large bowl, toss the mixed baby greens, cucumber, cherry tomatoes, and red onion together.
Thinly slice the rested steak against the grain.
Plate the salad, top with the sliced steak and avocado, and drizzle the lemon-herb vinaigrette over the top just before serving.