To Prepare the Beef: Using a food processor, pulse together the garlic, corriander roots, salt, black pepper and 2 tablespoons oil. Spread the mixture on the steak. Heat the remaining oil in a heavy bottomed fry pan. Cook the steak about 3-4 minutes on each side. Remove from pan and allow to cool. To Prepare the Salad: Slice the cooked steak into thin strips. Place the prepared lettuce on the serving plate and arrange the mango, cucumber, green onion and strips of steak on top. To Make the Dressing: In a small bowl combine the fish sauce, lime juice, soy sauce, red chilies and brown sugar. Stir until sugar is dissolved. To Serve: Drizzle the salad with the dressing and scatter cilantro leaves over the top.