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 desired doneness. Remove from the grill and let it rest for 10 minutes before slicing thinly against the grain.
In a small bowl, whisk together lime juice, fish sauce, sugar, and chili paste until the sugar is dissolved. Adjust the seasoning to taste.
In a large bowl, combine the sliced beef, cherry tomatoes, cucumber, red onion, cilantro, and mint. Pour the dressing over the salad and toss gently to combine.
Serve the salad over a bed of mixed greens. Enjoy immediately or chill in the refrigerator for up to 30 minutes for a refreshing taste.