Place the beef in a shallow dish. In a bowl, combine 2 tablespoons fish sauce and 1 tablespoon lime juice. Pour over the meat, turning to coat. Refrigerate for at least 3 hours.
Drain the beef and grill to your preferred doneness—rare or medium-rare works best. Rest for 5 minutes, then slice thinly.
In a large salad bowl, toss together the cucumber, chilies, green onions, tomatoes, mint, and coriander. Whisk the remaining lime juice and fish sauce with palm sugar, soy sauce, and garlic to make a smooth dressing.
Drizzle the dressing over the salad and gently toss. Divide among plates and top with sliced steak. Serve immediately for maximum freshness and flavor.
