Preheat a grill or grill pan over medium-high heat. Rub the flank steak with vegetable oil, salt, and black pepper.
Grill the steak for about 4-5 minutes on each side for medium-rare, or until it reaches your desired doneness. Remove from the grill and let it rest for 5 minutes before slicing it thinly against the grain.
In a large bowl, combine the salad greens, cucumber, cherry tomatoes, red onion, cilantro, and mint.
In a small bowl, whisk together lime juice, fish sauce, sugar, and chili flakes until the sugar is dissolved.
Add the sliced beef to the salad and drizzle the dressing over the top. Toss gently to combine.
Serve immediately, garnished with additional herbs if desired.