Prep your chicken. If you're using rotisserie chicken, pull all the meat off and chop it into bite-sized pieces. Discard the skin and bones. If you cooked chicken specifically for this, make sure it's completely cooled before you start.
Make the dressing. In a large bowl, whisk together the mayo, Dijon mustard, and lemon juice until smooth. The lemon juice keeps it from being too heavy and adds brightness.
Add the vegetables. Toss in the celery, red onion, and parsley (if using). The celery needs to be diced pretty small—big chunks are annoying to eat in chicken salad.
Add the chicken. Add the chopped chicken to the bowl and stir until the chicken is evenly coated in the dressing. Don't overmix, or you'll break up the chicken too much.
Season and taste. Add the salt and pepper, then taste it. You'll probably want to add a bit more salt—chicken salad needs more seasoning than you think. If it tastes flat, add another squeeze of lemon juice or a pinch more salt.
Chill before serving. Cover and refrigerate for at least 30 minutes before eating. This gives the flavors time to blend together and tastes way better than eating it immediately.
