Drain the tuna cans thoroughly, pressing with a fork to remove excess liquid. Transfer to a medium bowl.
Add the Greek yogurt, lemon juice, and Dijon mustard to the tuna. Mix gently with a fork, breaking up any large tuna chunks while maintaining some texture.
Fold in the finely diced celery and red onion, distributing evenly throughout the mixture.
Add the chopped fresh dill, stirring gently to distribute the herbs throughout the salad.
Season with salt and freshly ground black pepper to taste, remembering that the Dijon mustard already contains some salt.
For best results, cover and refrigerate for at least 30 minutes to allow the flavors to meld together.
Before serving, taste again and adjust the seasoning if needed.
Serve chilled as a sandwich filling, stuffed in avocado halves, or atop a bed of fresh greens.