Preheat your 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 5-10 minutes before slicing.
While the steak is resting, prepare the dressing by whisking together lime juice, fish sauce, soy sauce, brown sugar, garlic, ginger, and red pepper flakes in a small bowl.
In a large bowl, combine the mixed salad greens, cherry tomatoes, cucumber, red onion, cilantro, and mint. Toss gently to mix.
Slice the rested steak thinly against the grain and add it to the salad. Drizzle the dressing over the salad and toss to combine.
Top with chopped roasted peanuts if desired. Serve immediately.