Cook the Chicken (If Needed):Boil or bake chicken breasts until fully cooked (internal temperature of 165°F), then allow to cool and shred or cube.
Prepare Vegetables:Finely chop celery and red onions. Set aside.
Mix the Base:In a large mixing bowl, combine mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Mix until creamy.
Add Chicken and Veggies:Add shredded chicken, chopped celery, and onions into the bowl with the dressing.
Combine Thoroughly:Stir everything together until the chicken is well coated with the mayo mixture.
Add Optional Ingredients:Mix in boiled egg, parsley, or pickles for extra flavor if desired.
Chill and Serve:Let the salad rest in the fridge for at least 10 minutes before serving to let the flavors meld.