In a large bowl, combine the chicken, celery, and red onion. Give it a good stir so everything starts mingling.
In a separate small bowl, mix the mayonnaise, Dijon mustard, and lemon juice. Add salt and pepper. Taste it—if you like a bit more zip, add a touch more mustard.
Pour the dressing over the chicken mixture and stir until coated evenly. Don’t be shy—get everything nice and creamy.
If you’re feeling fancy, toss in some halved grapes, a chopped apple, or a handful of toasted almonds for crunch.
Chill for at least 30 minutes. Or eat it straight from the bowl—I won’t tell.
Serve it in lettuce cups, between bread, or just grab a fork. This salad doesn’t judge.