Wash celery stalks thoroughly under cold water.
Trim ends and remove any tough strings.
Slice celery diagonally into thin pieces.
Place sliced celery in large mixing bowl.
Whisk olive oil and lemon juice together.
Add Dijon mustard to oil mixture.
Stir in honey until well combined.
Season dressing with salt and pepper.
Pour dressing over celery slices evenly.
Add fresh parsley and capers to bowl.
Toss everything together until well coated.
Let salad rest for 10 minutes.
Garnish with toasted walnuts if desired.
Serve immediately or chill until ready.