Melt lime Jello in boiling water according to package directions. Add 1/2 the amount of cold water or ice cubes called for and chill until partially congealed. When Jello is cool, but before it starts to "set" add crushed pineapple (reserving 1/4 cup of the juice for later) and walnuts (keep back about 3 tablespoons nuts). Refrigerate until "made." Allow cream cheese to come to room temperature and blend well with reserved pineapple juice. Spread over top of Jello. Chop remaining walnuts until almost a powder and sprinkle on top of cream cheese. Cover with plastic wrap and return to refrigerator to completely chill. Cut into squares and serve on lettuce leaf.