In a medium bowl mix cilantro, mint, green onions, lemongrass, lime juice, fish sauce, sweet chili sauce, and sugar. Whisk until everything is well combined.
If the steak is not already cooked, cook over high heat on a grill for about 5 minutes on each side. Slice the meat into thin strips and let cool.
Add the steak to the bowl of marinade, cover, and put in the fridge for at least 3 hours.
Divide the lettuce into bowls. Top with tomatoes, cucumbers, and steak. If you want more dressing on your salad, pour the extra juice from the marinade on top.
Top with peanuts and fresh cilantro.