In a bowl, combine the chicken, mustard, mayo, celery, salt, and pepper. If making ahead of time, cover and keep chilled until ready to serve.
When ready to serve, slice avocados in half; remove seed and peel carefully. –keep the pits in a jar with cold water-
Slice a bit off the bottom halves so that avocados can stand without tipping over. Fill each avocado with chicken salad. Serve immediately.
Note: You can make chicken sandwiches with left overs salad.