In a mixing bowl, combine soy sauce, fish sauce, brown sugar, lime juice, minced garlic, and ground black pepper. Whisk until the sugar is dissolved. This marinade will infuse the beef with flavor.

Add the flank steak to the marinade, ensuring it is fully coated. Cover and refrigerate for at least 15 minutes, or up to 2 hours for more flavor.

Preheat your grill or grill pan over medium-high heat. This ensures a good sear on the beef.

Remove the beef from the marinade and discard the marinade. Grill the steak for about 4-5 minutes on each side for medium-rare, or until your desired doneness is reached. Use a meat thermometer for accuracy (130°F for medium-rare).

Once cooked, remove the steak from the grill and let it rest for 5 minutes. This allows the juices to redistribute, keeping the meat tender.

While the beef is resting, prepare the salad. In a large mixing bowl, combine the mixed salad greens, cucumber, cherry tomatoes, red bell pepper, red onion, cilantro, and mint. Toss gently to combine.

In a small bowl, whisk together lime juice, fish sauce, sugar, minced garlic, and chopped red chili (if using). Adjust the seasoning to taste.

Slice the rested beef thinly against the grain. This ensures tenderness.

Top the salad with the sliced beef and drizzle with the dressing. Toss gently to combine, ensuring everything is well coated.

Serve immediately on a platter, garnished with additional herbs if desired.