In a large bowl, combine mayonnaise, Dijon mustard, lemon juice, garlic powder, salt, and pepper. Stir until smooth.
Add the cooked chicken, pecans, celery, red onion, and parsley. Stir until evenly coated with the dressing. Taste and adjust seasoning as needed.
For the best flavor, cover and chill for 30 minutes before serving.
Notes
Watch the salt if using store-bought mayo or rotisserie chicken. Both can already be seasoned, so taste before adding the full 1/2 teaspoon of salt and adjust from there.
Rotisserie chicken works great here. Just remove the skin and bones before chopping. The skin can make the salad greasy, and it won't chop evenly.
If your salad looks a little separated after chilling, just stir it back together. This is normal; the mayo can loosen slightly as it sits.
Want it spicier? A pinch of cayenne, a few dashes of hot sauce, or some diced jalapeño all work well without changing the texture.
Bold move: the lemon juice and chill time are the two things that make this salad taste like more than "mayo and chicken." Don't skip either one.