Preheat broiler. Place sausages in one layer on a large baking sheet and broil until charred on all sides, about 5 minutes per side. Set aside to cool.
To make onion cups, slice off root ends and tops, then peel. Cut each in half horizontally, and separate layers by pressing with your thumbs against root end to release rings. Reserve 12 largest as serving cups. Slice remaining onion into thin strips for use in salad.
Slice cucumbers in half lengthwise. Then slice cucumbers and charred sausages, diagonally.
Combine sausage, cucumbers, onion strips, celery leaves, and cilantro in a medium bowl. In another bowl, whisk together serrano chiles, lime leaves, garlic, palm or brown sugar, lime juice, and fish sauce. Pour over salad and toss to combine.
To serve, place onion cups openside up on each serving plate. Spoon salad into cups and serve.
c.1997, M.S. Milliken & S. Feniger, all rights reserved