In a medium bowl, beat cream cheese.
Add yogurt and mix until smooth.
Stir in celery, mustard, chives, salt, and grated lemon peel and juice; mix until blended.
Stir in chicken.
Cover and chill until ready to serve.
Rinse avocados. Cut in half.
Gently lift out the pit with a spoon.
Spoon out the avocado, then add about 1/3 cup of the chicken salad into each avocado half.
Garnish with chives.