Place diced red potatoes in a pot of salted water, bring to a boil, and simmer for 10-12 minutes until fork-tender.
Season chicken breast with a pinch of salt and pepper, then grill or pan-sear until the internal temperature reaches 165°F.
Place the egg in boiling water for 9 minutes, then transfer to an ice bath before peeling and chopping.
In a small bowl, whisk together the Greek yogurt, Dijon mustard, olive oil, lemon juice, sea salt, and black pepper until smooth.
Dice the cooked chicken, potatoes, and egg into bite-sized pieces and place them in a large mixing bowl.
Add the finely chopped celery, red onion, and fresh dill to the bowl.
Pour the Dijon vinaigrette over the ingredients and toss gently to combine, ensuring everything is evenly coated.
Serve immediately or chill in the refrigerator for 30 minutes to allow the flavors to meld.