Marinated feta makes this salad something special. Flavorful flank steak has a lot more going for it than just protein. It’s also high in selenium, zinc, and niacin.
Whisk together mint, lemon juice, 4 tablespoons of the oil, and 1 tablespoon of the oregano in a small bowl. Pour into a zip-top plastic bag; add feta cubes, and seal bag. Let marinate at room temperature 20 minutes, turning bag occasionally. Remove feta from marinade, and set aside; reserve marinade in bag.
Season the steak with the pepper, 1/2 teaspoon of the salt, and remaining 1 tablespoon oregano. Heat remaining 2 tablespoons olive oil in a 10-inch cast-iron skillet over medium-high. Add steak to skillet; cook, undisturbed, about 3 minutes per side for medium doneness. Remove from heat and let rest for 5 minutes before slicing across the grain.
Toss together spinach, cucumber, and basil in a large bowl with reserved marinade. Arrange mixture on a platter; top with feta and steak. Sprinkle with lemon zest and remaining 1/4 teaspoon salt.