Preheat the grill to medium-high heat. Season the flank steak with salt and pepper, then grill for about 4-5 minutes on each side or until desired doneness.

In a mixing bowl, whisk together fish sauce, lime juice, sugar, minced garlic, and chili flakes until well combined.

In a large bowl, combine mixed salad greens, cucumber, cherry tomatoes, red onion, cilantro, and mint. Slice the grilled beef and add it to the salad.

Drizzle the dressing over the salad and toss gently to combine. Serve immediately.