Preheat a grill or grill pan over medium-high heat. Rub the flank steak with vegetable oil, and season with salt and pepper. Grill the steak for about 4-5 minutes per side for medium-rare, or until desired doneness. Remove from the grill and let it rest for 5-10 minutes before slicing thinly against the grain.
In a large bowl, combine the mixed salad greens, cherry tomatoes, cucumber, red bell pepper, cilantro, mint, and chopped peanuts.
In a small bowl, whisk together the lime juice, fish sauce, sugar, garlic, ginger, and red pepper flakes until the sugar is dissolved.
Drizzle the dressing over the salad and toss gently to combine.
Top the salad with the sliced steak and serve immediately.