Finely chop the scallions or green onions including a few green tops. Oranges should be pared and sectioned, or can be unpared, cut into wedges. Avocados should be pared and cut into wedges. Sprinkle the scallions with the lime juice and 1/4 t salt; cover and refrigerate. Mix the remaining ingredients except the lettuce, oranges and avocados and refrigerate at least 1 hour. Spoon chicken mixture onto lettuce. Garnish with oranges and avocados; sprinkle with scallions.