Combine the mayo, Dijon mustard, and dried dill in a large bowl. Season with salt and black pepper, to taste, and stir to combine.
Add the shredded chicken breast and chopped celery to the bowl and toss to combine.
Divide the baby spinach between 4 chilled serving plates. Add one-quarter of the chicken salad to each plate and top with some blue cheese and chopped walnuts. Serve immediately. Enjoy!