Season chicken breasts with salt and pepper, then poach or grill until internal temperature reaches 165F (74C).
Allow chicken to cool completely before cubing into 0.5-inch pieces or shredding.
In a large mixing bowl, whisk together Greek yogurt, Dijon mustard, lemon juice, garlic powder, salt, and pepper until smooth.
Add the prepared chicken, diced celery, minced red onion, and fresh dill to the bowl.
Fold the ingredients together until the chicken is evenly coated in the yogurt dressing.
Divide the salad into four airtight containers.
Store in the refrigerator for up to 4 days.