Add the red onion and vinegar to a small bowl. Allow the onion to soak in the vinegar while are are preparing the rest of your ingredients.
Toss the chopped chicken together with the celery, parsley and tarragon in a large bowl.
Add the sugar, salt, lemon pepper and mayonnaise to the bowl with the vinegar and onion. Whisk until well combined.
Pour the mayonnaise mixture over the chicken and stir well to combine. Cover the bowl with plastic wrap and allow to chill for about an hour before serving.
This recipe can easily be multiplied to serve more people.