If poaching, place chicken breasts in a large pot and cover with water or chicken broth. Bring to a gentle simmer and cook for 15-20 minutes, or until the internal temperature reaches 165°F (74°C). Remove from the pot and let cool. (Alternatively, use shredded rotisserie chicken or leftover cooked chicken).

Once cool enough to handle, shred the chicken into bite-sized pieces using two forks or chop with a knife. Place the shredded chicken in a large bowl.

In a separate small bowl, whisk together the mayonnaise, Dijon mustard, and lemon juice until smooth.

Add the diced celery, red onion, and chopped fresh dill to the large bowl with the shredded chicken. Pour the dressing over the chicken and vegetables. Season with salt and pepper.

Gently stir everything together until well combined. Taste and adjust seasoning if needed. For the best flavor, cover the bowl and refrigerate for at least 30 minutes before serving.