If you are starting from dry pasta, boil a large pot of water and cook the pasta to the package directions.
Put the cooked pasta in a bowl. Add the chopped green onions to the pasta, then add the mayonnaise. Add the chopped chicken and mix well.
Remove the skins from the avocados, cut them in half and remove the stones. Cut 8 thin slices for garnish and chop the remainder of the avocados. Sprinkle the sliced avocado with lemon juice and set aside. Add the chopped avocados and tomatoes to the pasta and mayonnaise mixture with the cooked chicken. Season well and add a few drops of lemon juice.
Arrange the salad in a bowl which has been lined with large lettuce leaves. Decorate with slices of avocado. Sprinkle with chopped nuts if desired. It adds a nice crunch and flavor.