In a large pot simmer the chicken breasts, in enough water to cover, with the celery onion, salt, and pepper, until tender. Remove from the stock and cool. Debone and shred into bite-size pieces.
Strain the stock and refrigerate for another use. Combine the chicken, mayonnaise, soup, soup base, and curry powder. Place a few avocado slices in each lightly greased individual au gratin dish. (You can also bake this in a 13 x 9 baking dish or even a casserole dish.) Pour chicken mixture over the avocados and top with the cheese. Bake about 20 minutes at 350 degrees or until bubbly.