Place the eggs in a small pot of water, bring to a boil, then cover and remove from heat for 10 minutes before cooling in ice water.
Season the chicken breast with a pinch of salt and pepper, then poach or pan-sear until cooked through and shred into bite-sized pieces.
Peel the cooled eggs and roughly chop them into small chunks.
In a medium bowl, whisk together the Greek yogurt, Dijon mustard, lemon juice, dill, chives, garlic powder, salt, and pepper.
Finely dice the celery and red onion.
Fold the shredded chicken, chopped eggs, celery, and onion into the yogurt dressing until well combined.
Toast the sourdough bread and top with the chicken and egg salad mixture.