Place eggs in a saucepan, cover with cold water, bring to a boil, then remove from heat and let sit for 12 minutes before cooling in an ice bath.
Peel and roughly chop the cooled hard-boiled eggs.
In a medium mixing bowl, combine the drained tuna, chopped eggs, celery, and red onion.
Add the Greek yogurt, Dijon mustard, and lemon juice to the bowl.
Fold the ingredients together gently until evenly coated and well combined.
Season with salt, black pepper, and fresh parsley.
Serve immediately or refrigerate in an airtight container for up to 3 days.