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.
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.
In a small bowl, combine all the ingredients and stir until the sugar is dissolved.
Drizzle the salad with the dressing and scatter the reserved cilantro leaves over the top.