To Prepare the Lamb: In a food processor, combine the garlic, cilantro stems, salt, pepper, and 2 tablespoons oil. Spread the paste on both sides of the lamb and marinate for ½ hour. Warm remaining oil in a large, heavy skillet, cooking the steak on each side 3 to 4 minutes for medium rare. Remove from pan and let cool. To Prepare the Salad: Cut the lamb into strips. Divide the lettuce leaves among 4 plates and arrange the mango, avocado, green onions, and strips of lamb on top. To Make the Dressing: In a small bowl, combine all the ingredients and stir until the sugar is dissolved. To Serve: Drizzle the salad with the dressing and scatter the reserved cilantro leaves over the top.